Is there a size limitation beyond the underlying filesystem for the default summary index? If so, how big can it grow?
The shipped maximum is 500,000 MB (about 500 GB), subject to lower limits imposed by the size of the file system that hot/warm and cold paths are on. You can raise these, as well as restrict how much is written to the hot/warm or cold filesystem by configurations in indexes.conf.
And note that in general you may want to create new indexes for summaries. There is nothing fundamentally different about a summary index vs a regular index.