I am in the midst of trying to design my own wedding invitations and
have run into an unexpected problem:

We would like to print onto some cards we purchased from a paper
store, designed to fit in an A6 envelope. They measure 4.5" x 6.25" .
We have made our own image in photoshop, but when we try to print it,
it leaves borders on the left and right edges.

When choosing the paper size in printing properties, there are a
couple options for borderless photos, but the only ones available are
the incorrect size (either 4"x6" or 10cmx15cm). From playing with
these options and reading other posts I have determined that the
printer is physically capable of printing to the edge on 3 sides
(which is luckily all I need).

So, if the printer can print to the edge on a 4x6" piece of paper, why
can't I do the same for one that is 4.5x6.25"?? Is this a limitation
in the printer drivers? Do I just need to wait around until they fix
this? Or am I missing some obscure option that would allow me to do
what I want?

This is frustrating to say the least. I would appreciate any help that
anyone could offer, including recommendations for printers that can do
what I want.

Thanks in advance!