In the width property field for the “Collection Item” div. See screen grab.
You can change the fixed pixel width on “divblock 8” but they won’t be perfectly square. Really depends on how you want them to look.
I’ve added another screen grab below. You’ll want to remove any float changes completely and keep the default (which is float left). I apologize, in my previous post I said to remove float… but I should have said to click on the blue word “float” and then click “remove this style”