Any reason to run bigger than 1" pex to water to air coil

Just a guess here but if the w/a hx is plumbed with 1" then the mfg stated btu capacity is probably rated with the 1" flow capacity in mind. Deviations to that rating would probably best be effected by a circ with multiple speeds and higher head pressure than the hx was rated for the given btu's. So unless you were planing changes in the future it sounds like an un-necessary expense.

Based on what I've read here and other places your 1" pex will NOT move anywhere near your coil rated capacity.

I'm running 1-1/4" Pex to my HX which is much smaller than 115k. Like Cave said, what are your inlet/outet sizes on the HX?

I'll first state that I'm not a professional. With that being said I will spew a few things anyway. Your Logstor 31mm (just over 1" pex) can handle between 38,000BTU and 75,000BTU with a flow velocity of 2ft/sec and 4ft/sec velocity without pipe wear/noise and with a 20deg delta-T. I think the key might be your delta-T and or your blower cfm. If your design allows for a 20deg delta-T then your stated heat loss of 48,000BTU is considerably lower, heck, even your rated furnace output is lower at 68,000BTU than what your pex 'allows' at 75,000BTU. If however your design is say a 10deg delta-T then you would need 12gal/min and no...your pex is too small for that. I'll assume your design is 20deg delta-T since that is the industry standard in North America. Now the question is what is your flow rate of your pump? Doing some quick calculations for your head loss (using some equivalent length numbers) you should be able to achieve a nice flow rate with a Taco 005 or 007 or equivalent and meet your 6.8gal/min flow target assuming again a delta-T of 20deg. Using a 20deg delta-T 1GPM=10,000BTU's. I'm using the higher furnace load for safety although I would want an accurate heat loss of the space your intending to heat. Even at that, your 1" Logstor can handle 75,000BTU's and be within the 4ft/sec parameter.
Probably your most important factor is finding the output ratings of your particular water-air exchanger. I did a quick search and found this on the web to use as an example. http://www.ctwoodfurnace.com/PDF/V101.2 VAL Air2Water_logo_pg2.pdf Your particular exchanger might vary so I would definately find out about your own particular models specs. I think you might find that the cfm of your furnace blower is not giving you enough velocity to achieve your requirements using 130deg entrance water into your exchanger. Your particular exchanger is nicely oversized to take advantage of lower temp water so that leads me to believe your not blowing enough cfm across it or your flow rate is too low through it but that seems impossible with the dinky size pumps you need. If if were me there is no way I would change out your 1" pex unless your heat loss was over 75,000BTU's your design is for a smaller delta-T and or your headloss was so high your wire to water was so inefficient that you couldn't find a pump that could pump your required flow. In your case it seems like that at least is not the case, assuming again a design of 20deg delta-T. I know I know delta-T again but it is very important in this case.
If however you need more than 7.5GPM flowing through your exchanger than yeah, you would probably want larger pex. But you could even flow 9GPM through that pipe (90,000BTU's) for short periods of time say when your water temp is lower. Or blow more cfm across your heat exchanger. That I would definately find out about.
I know I'm spewing and again I'm not a professional but I would seriously look at all the design parameters before I changed out my pex. Because in your case everything looks good (assuming a 20deg delta-T ).

Good luck, and maybe a professional will chime in and really actually help you...I'm just spewing you see.

Maybe but maybe not. You need to know the specs of your particular exchanger because it also might not matter if you use bigger pipe. Without the specs it's hard to 'guess' because the exchanger has to be sized appropriately to use say 110 deg water at what flow rate with what cfm in order to determine a particular BTU output. If it doesn't have enough surface area to output your needed BTU's at say 110 deg water than it won't matter what size pipe you use. You should be able to find out the specs for your particular model from the manufacturer so that you know how many btu's you can extract from it at a specific entering/leaving temperature for a particular flow rate and a particular cfm.
But with that said if you have to replace the pipe anyway go as big as you can afford because the flow rate of a properly sized exchanger to take advantage of 110 deg water for your needed BTU's might be quite high.

Quote from idronics july 2010 vol 8 page 7...."It might seem intuitive to assume that heat transfer from a heat emitter increases in proportion to flow rate through it (i.e., doubling the flow rate through the heat emitter would double its heat output). However, this is NOT true."

It's a non linear relationship. Therefore to take advantage of lower temperatures you need more emitter surface area as in the example of slab radiant. Your heat exchanger might have enough surface area for what you want to do but just increasing the size of the pipe to it might have no effect at all if your surface area of your heat exchanger isn't large enough.

Hope this helps somewhat.

To put what BackBurner said another way..... You are probably limited by the air side of the HX, not by the water flow. At higher water temperatures, there is a larger temperature difference between the water and the air, so you get more heat transfer. As water temps go down the air temperature remains roughly the same, decreasing the temperature difference between the water and the air and decreasing the heat transfer to the air. Having a higher water flow rate will not make much, if any, improvement. Increasing the surface area on the air side of the exchanger is likely the most effective. Increasing the air flow rate may or may not be effective depending on the exchanger and the current air flow rate. This is a simplification, there are a lot of non linear factors that go into heat transfer on both sides of the exchanger. I am making the assumption the exchanger is approximately sized for the application with 160 to 180 water temps.