Need advice - MTB frame size for stem-riser build
 

Hi, I'm building up an upright MTB bike for my father, who has a herniated disk in his back. All the upright/MTB setups we've tried in shop have been too low and have aggravated the injury.

So my strategy now is to add a stem riser/extender to a steel hardtail MTB off craigslist - Bridgestone M-series, Trek Antelope, Specialized Rockhopper/Stumpjumper, Giant Iguana, etc.

He's 6ft tall with a 31" inseam. According to guides I'm reading online, his MTB frame range is 18.5-21.5 inches. My question is, given that I'll be adding extra stem length for this upright build, is it better to attempt this set-up on a frame in the smaller range (18-19in), middle (20in) or larger (21-22in). I don't know enough about frame geometry to estimate how the added stem length will effect top tube + stem reach length.

I hope this is enough to go on. Thanks for your thoughts.

Moved from Mechanics to General Cycling.

In this situation, you want to have the head tube as high as you can get it and a short top tube. But you don't really want to increase the standover distance too much. Some of the newer comfort bikes fit this better than the older bikes you have described.