I'm guessing you mention size because the Inspire is smaller. Honestly, the Infuse FEELS smaller to me. The Infuse is a little bit taller, it's width dimensions are nearly the same, and it's quite a bit thinner which gives it a smaller feel to me. I find the Infuse much better feeling in the hand than almost any 4.3" phone due to it's thin lightweight design. Being that it's only a tenth of an inch wider, I don't think it would make a whole lot of difference in texting for her.
Most important factor is you said flashing back to stock for her. This is where a HUGE warning flag pops up. HTC's Sense UI is absolute dog **** on that phone. It crashes frequently, it slows the phone down incredibly. That phone has turned more people to iPhones than any other Android phone I know. If you'd be running stock software, then I HIGHLY suggest you avoid the Inspire. Sense used to be something you WANTED on a phone, and Sense 4.0 looks like it's getting better again, but for a while there I wouldn't have taken a phone with a Sense based UI if you paid me. If you're willing to put a custom ROM on it for her, then the Inspire has some decent ICS ROMs as dman mentioned, and probably even pretty good Senseless GB ROMs, but stock, no way. It reminds me of the original stock version of Eclair on the Captivate. Slow as dirt. At least that's my opinion and the impression I got from using several friends' Inspires.