There is no overall repository size limitation in Assembla.
However, due to general SVN design specifics, we apply some strict rules, to prevent one repository from generating excessive load and affecting other customers on our servers.
We place strict maximum file size limit: 500MB.
We also place strict limit on amount of files per commit: 5000 files.
Another SVN weakness is high RAM consumption when processing changesets with large number of files. To keep our file system healthy and our customers happy, please split your commits into smaller chunks.
You may also be interested in Assembla's Private Installation on dedicated hardware, which is free of the limitations mentioned above.