If you have a good table saw, cutting the cabinet is not too hard. Cutting the depth is not that hard, cutting the width is best done with a new faceframe. Modifing the faceframe is hard to do so it does not look modified.
If it has drawers, it can get a little more complicated. Why not built one to size??
How much do you need to remove? Are we talking about 1/8"? 1/4"?
Most cabinets have about 1/4" of edge on the face frame...they do this so that the face frame will sit flush agains the wall and you don't have to worry about the sides (if they are wood) preventing you from squaring up the cabinet.
As you will find out, the 1st rull of home improvement is "Walls are not straight".
2nd Rule....it will always cost more than you plan......etc, etc...
If you don't need to take up much, you can sometimes trim the edges.
If we are talking about chopping off a few inches.....no easy way to do it without it looking ugly.
"The dream is free but the hustle is sold separately."