Hi Terri, the 600x600 is the preferred one, unfortunately the size showing over on the right hand size came with the software and we have no way of changing that unless we get the software customised, which costs a lot of money.
However, having said that, in recent years we have seen the emergence of 800x800 which is okay as well BUT for that size you need to keep the file to under 200kb (the larger size does take up more room on the servers which is why we need to keep them under the 200kb limit).
Sorry for the confusion Terri! I have meaning to put that in the gallery guidelines for ages but I just keep forgetting. I will update them tomorrow though.