I would go with as large of an insert as you can fit (reasonably) in the space while still allowing 3-4"+ of clearance from the stovetop to lintel to be able to secure the liner to the output. Longer burn times are nice & if it ends up overheating the house you can always open some windows upstairs ;-D
Much better than finding out later the insert is too small and the house is cold.
Here're some of my research notes from looking at chimney liners:
Stainless Steel Alloys (cheapest/worst -> best): 304, 316, 316Ti
Liner thicknesses: 0.005" - 0.015"+
- stay away from "smooth wall" type double-laminated liners as the two layers may separate when bumped around during installation
- 0.005" thin 316Ti may be just as durable as a 0.010" 304 alloy liner as long as no metal brushes are used to sweep
- pretty much all liners will have a "lifetime warranty", but if the liner was damaged during installation it likely won't be covered (don't kink/dent that 0.005" steel!)
- that extra-cheap 0.005" 304 liner has a "lifetime" warranty, but it's not going to cover paying for removal/reinstallation if it fails...
- not all have a fully transferable warranty to the next homeowner
- warranty may be void if not swept/inspected yearly
- get the full 1/2" insulation blanket wrap kit unless the chimney is fully inside the home
- get a bunch more stainless hose clamps or stainless wire wrap to hold the insulation on in addition to the spray adhesive, especially if it's going to be a tight fit in your flue (measure!) or there's a large bend to one side
Installation notes:
- may need a building permit
- home insurance may drop you for DIY installations, find out first what they need to keep coverage!
- may be able to DIY install and have a certified technician inspect things later, but most insurance requires professional installation
- make sure any quotes you get include a breakdown of what types of liners/parts they will use to help compare the true cost of the cheap vs. expensive companies