Can I resize a bench
If the benches that you have are indeed the Norden style of benches from IKEA like this:



Then it would be possible to shorten them slightly by cutting an equal amount off of each end if the bench top. Several things to consider when proposing to do this:


Some IKEA products are made of a particle board that is veneered with a nice wood face and edges. If the top boards of this bench are made that way then the shortening would leave ugly raw edges on the ends of the bench.
Cutting wants to be uniform from both ends so the bench remains having a balanced and even look. 
Do not shorten so that the top gets closer than say 1 inch (2.5 cm) from the legs.
If the ends of the top are cut it will leave a raw wood edge that would need to be re-finished. Good guess as to what type of finish has been used by the manufacturer.
If you cut only one of the two benches they may no longer look like a "matched set".
The type of saw used to trim the bench top will have a lot to do with how nice the cuts look and whether or not there is any chipping or splintering.