I'm trying to make some designs with picmonkey and it will not let me, for the measurements
I use picmonkey sometimes. Do you use a Mac or PC? I've seen tutorials where designers use picmonkey's 8x10 template on their Mac and then drop their designs into Merch's template. As for me, I use a PC, so I open a custom canvas design at 3400 x 4080 px, save the design with a transparent background, then upload my .png file to a resizing software and resize to the 4500 x 5400 px required by Merch. It's a 2-step process which can be a pain, but works for me. However, if yours is a Mac computer I believe it's an easier process without that extra step. Hope this helps. Everything I learned on designing for Merch was from YouTube videos. And I'm still learning after 2 years. Good luck to you!