Factory 870 forearms come in two basic lengths- a 9" (give or take a little bit) 'field' length, that covers part of the loading port when retracted, and a 7" 'police' forearm, which leaves the loading port uncovered when retracted.
Then there are a raft of aftermarket alternatives, as well as a couple of styles of fancy schmancy trap or skeet forearms too. So there are lots of options.
Personally I like the factory 'police' length forearm in either wood or synthetic, given a choice. It allows easy access to the loading port for admin unloading by manipulating the shell stop, and it allows the operator to use the firing hand thumb as a magazine cutoff if needed without pinching said digit. And it lets you use a six round Sidesaddle.
But my wife has problems reaching a 'police' style forearm, so the house guns here have 'field' length forearms installed to go with their shortened sticks. Oh well. They fit her and I have learned to use them.
The nice thing is there's only one size fore-end tube for Remington shotguns, unlike other makes. So your choice of forearms (in the proper gauge) is a one-size-fits-all proposition.
hth,
lpl/nc