what is the difference in floor heights? An accessible ramp is 12" in length for every 1" of height (2009 IRC will allow 1:8 instead of the normal 1:12 under difficult conditions). Most codes require a minimum of 80" of head room ....... may want to check out an electric lift depending on height, many times you can find used ones fairly inexpensive.
don't know if you'll find plans online. I typically build ramps with 2x8 joists at 12" o.c., spans are based upon wood species and grade and loads on the ramp.
"You get what you pay for, and sometimes free costs more!"
I was going to say. I think you will end up with a slide, not a ramp. Really sorry for your wife. Pain is not fun. My wife and I both have Arthur and other stuff. Sucks, and no doctor wants to give you the meds you really need.
" A lot of men build things, and a lot of things fall down "