Even assuming that the printer could handle custom sizes (not apparent), envelopes vary widely in thickness and the type of paper used to make the envelope. An envelope with a foil insert (for example) might not feed properly.
If you can stand the idea, consider printing nice labels for the envelopes. Labels come in a variety of shapes and sizes - these can be decorated with tiny images and fancy fonts. Best (to) stay away from "glossy" labels as these might produce unexpected results.