Whether you get a frame with or without a mat, we will cut the frame such that the artwork size you provided is covered by 1/4" on each side. This is so that the art doesn't "fall through" the matting, or in the case of a frame without matting, show white borders around the art.
If you have artwork with important details (such as an artist signature, or visually important detail) around the outer edge that you don't want to cover, the best you can do is give our website an artwork size that is 1/4" larger in each dimension than your art. So for example, if your art was 10" x 10", you would use an art size of 10 1/4" x 10 1/4". This way, we'll send you a frame that only covers the artwork by 1/8" on each side, which is the smallest overlap possible without the artwork revealing the backing behind it on at least one side, or "falling through" the mat opening.