What are the best image sizes to use when printing large? Im getting a little confused working out the aspect and whether the photo will be cropped or not?
What's already been said, basically! Unless you print at home on A series paper sizes, in which case either crop the original to suit those sizes (1:1.41 is close enough, or use the exact paper dimensions as stated on the packaging) or calculate the biggest size that'll fit the paper you're using. I usually crop to fit the paper I'm using unless I'm aiming for a specific frame/mount.
If your camera shoots 3:2 aspect ratio, then those will print at 6x4, 12x8 or 24x16 inches without any need to crop.
If it shoots 4:3 aspect ratio, then those will print at 8x6 or 16x12 inches without any need to to crop.
It will say in your camera manual whether it's 3:2 or 4:3; or post your camera model here and I'm sure someone will know.
I'm still using Aperture, and when printing it has quite a nice feature where I can crop to the proportions I like (eg 8*10), click command-P, choose the paper size (eg A4), and the choose maximum to fit (need to uncheck a box or it crops it to A4 proportions which I usually don't want). Seems to work quite well...