First the 870 is a very good gun. I'm not a Remington fan, but they do have good 'modern' designs. The trigger unit is simple compared to other makes. You could always try a few shots at the range. I wouldn't punish yourself. Put on a recoil pad if you feel the need.
I always troed to fit the length so that there was 1 to 1-1/2 inches from where the thumb 'Y's into the hand to the tip of your nose. I usually did 1 inch so that some amount of clothing would take up the other 1/2 inch. If cold weather was expected, then taht amount of clothing was compensated for.
I only had one customer who had problems with it. He must have learned from someone or with one of the old, old double barrels with what was know as a crooked stock. His head rest near the butt of the stock. With black power shells the recoil wasn't as much as with modern shells. So this worked well back then. I had him try drifferent guns in the store to see what fit him.

Tom