We noticed that the device generator in magic allows a maximum size of 30um*30um for the MIM caps. But if we manually increase the size of the capacitor there aren't any DRC errors. Is there really a maximum size for this kind of capacitor? If there is, what would be the reason behind it?

I have been surprised at the lack of rules in this process for maximum metal width. All other similar processes I have dealt with have a maximum metal width (without slotting), and that width is usually around 30um. Metal under MiM caps cannot be slotted, so such a rule would limit MiM cap side. Despite the lack of explicit rules, I wanted to limit the MiM caps generated in Magic to what I consider a reasonable area for reliability. You can always edit the cell after it has been generated and resize it to whatever you want.