As Adrian suggested then. Create a new image with the dimensions of a standard print, e.g. 10x8 or 12x8 (12x8 would give you some white space for mounting on all edges) and then drop the image you want printed onto the centre of it. Trim down as required when you get the physical print through.
(if anyone comes across this thread at a later date and uses Lightroom, the easy way with LR would be to resize for printing and then add a border with mogrify to bring the file up to a convenient printing dimension).