Right now Mintbase is paying the Arweave AR token behind the scenes, so every time you upload and image, movie, or token metadata, a little gets taken at a time. There is no file size limit on the Arweave side, we just need to figure out how to pass this cost to the users before we increase the limits as the larger the upload, the larger the cost. Now that we are launched we can start down this road and we'll let you know as soon as we figure it out.
You are uploading your data to a hybrid, decentralized network that is daily new to this works so expect some failures. Before listing you item wait about 10 minutes (time to mine the data) and click the "Transaction Id" on the thing page if you want to confirm it was mined.