I currently offer 2 print sizes available to buy.
These are A4 and A3
A4 = 210x297mm
A3 = 297x420mm
Please note that a full frame digital camera produces images at a ratio of 2x3 (4x6).
For this reason I print at A4 and A3 as these sizes need the least amount of cropping and are the closest to the original ratio. Even on these sizes a slight crop to the sides of the original photo is needed. See example below.
I have noticed that A4 frames and mounts are easily available from most shops.
True A3 frames and mounts are a little harder to find but I found a good selection online.
In most cases an A3 print will fit in a standard 300x400 mount, however it does crop the sides a little more than a true A3 mount.
Below are examples of how the the crop factor changes the image size.
Original Full Frame image
A4 and A3 Crops
300 x 400 mm crop (12 x 16 mount)