No it will not work. Very dangerous to even try.
A 255/60 looks pretty good on 8". I've used 275 before but that's about the limit. Go to an online tire dealer like tire rack. Put in some figures for tires. Click on the specifications for that tire. It will tell you the recommended wheel width.
A 20 inch tire.
No!. It is very unsafe to mix tire and rim sizes.
No. Even if you were able to stretch the tire on the rim, it will cause damage to the tire.
Yes, 17 inch rims will fit.
...............??....... NO
You destroy the tire trying to stretch it around the rim.
Rim size is important since you can't put a sixteen inch tire on an eighteen inch rim. But tire width is important too. Don't try to put wide tires on narrow rims. If you have questions, your local tire professional would be a good resource. Perhaps he/she would be impressed that you're a "chick".
No. Even if you were able to stretch the tire on the rim, it will cause damage to the tire.
what is the largest tire I can put on a stock 20 inch 2008 dodge ram rim
If the tire is built for a 17 inch wheel, yes.
No. You will not get the tire on the rim without tearing it.