Here's what's written on our wiki regarding what we suggest for Token specs for Marketplace content : "Tokens: 280x280 pixels in PNG format, regardless of the "in-game" size of the token. At this image size, the token could be as large as 4x4 squares in-game without any issues, and will support zooming in -- and it will look good on the Marketplace listings page, as well. Do be sure, however, that your tokens are still recognizable/"look good" at smaller sizes (e.g. 70x70 pixels). Note that tokens should "fill" the square without any excess padding, and feature a drop-shadow or other outline to make them easy to distinguish from background elements."
