Thanks for reaching out to us! Although you can’t currently enter the exact dimensions for a collage, you can drag the edge of the collage to resize it. If you watch the dimensions (as displayed down the very bottom right of the screen) you should be able to get very close to 1280 x 720. Unfortunately it’s sometimes tricky to get it exactly right, however this is something we will be improving in the future.
Let us know if you have any other questions!
Regards – The Ribbet Team