Well the way I do this when ordering online and as advised by the print lab is as follows;
First take a look at available sizes offered by the print lab.
So for example if they offer 8x6, 9x6, 10x8 etc
Decide what size you want your image to be on the longest size, so lets assume 8 inches.
Convert your image size in your editing software to 8 inches on longest side and at the resolution required by your print lab.
Lets say that works out at the odd size of 8 x 3.8 and 300ppi
So if you want some white space round the image then look at what they offer paper wise and the next one up that it will fit is 9 x 6
So if you have photoshop, go to canvas size and extend the canvas to 9 x 6. This puts a white border around the image but does not affect the image itself
If your printer has a profile then convert to this and send to the print lab
This means your print will come back as above and you will need to trim the short size to how you want it.
If in doubt ask your print lab how to do it, if they cant or wont tell you then find a new print lab.
Hope that makes sense