Well, if you used 1" pex let's see how the numbers work out:
1" inside diameter is about 9.4 cubic inches of water per foot of pipe.
At 231 cubic inches per gallon, that means it would take about 25' of pipe to hold a gallon.
That works out to 25,000' to hold 1000 gallons of storage.
I'm thinking that would be one whacking big pile of pex! On the other hand, that would be almost enough to heat my house from your boiler, so maybe it's a good idea after all