Hi, Aimee. The 1x8 dimension refers to the general rough size that the wood is sawed to on the sawmill. It is then dried and planed to 3/4" x 7-1/4" and sometimes this runs wide--up to 7-1/2". The commercial lumber buyers and sellers base their board foot pricing on the 1x8 size even though the actual size is much smaller after drying and planing.
Since this is a subfloor, gaps between the boards are not much of a big deal. You want to cut the broken boards half way over the nearest joist and fill in. No need to replace the whole board unless you want to. Since you will have to buy a longer board from your home center, try to find one where you can cut around any knots or other defects so grade should not be much of an issue. The wood was dry inside when your home center bought it and if it was going to move (cup, twist, etc.) it would have done it when your home center brought it inside and cut the bands on the bundle.
Since you have the subfloor exposed, mow is a good time to check for and fix any squeaks in the floor. A squeak is usually the wood rubbing against a nail or another board. Walk all over the subfloor and see if you have any spots that squeak. When you hear something, try to determine which board is moving to cause the squeak. Put 1-5/8" drywall screws through the board that is moving and into the joist. You will want to drill and countersink, particularly on the ends of the boards. You should use these for installing the replacement boards too.