I am aware of no limits with Thunderbird, but 30gig is still a lot for your PC to manage.

I have many more emails. My sneaky plan is to move messages older than three years to local folders. Then, through imap, I will only manage the three-year period.

My sneaky suggestion is don't. Mass moving of emails from IMAP to local storage is something of a mess. IMAP was just never really intended for moving gigabytes of mail at a time, and Thunderbird implementation really does leaves some things to be desired with regard to timeouts when large numbers of simultaneous changes occur.

Label all your GMail older that you desired date preferably more than one label perhaps one for each year. Use the Google takeout service to get that label/labels as an MBOX file. Note GMail labels all mail in the thread, so duplicates are quite possible accross mbox files.

Place the MBOX file, once unziped and with the file extension removed, into the Thunderbird local folders in the profile folder.

Start Thunderbird and select the folder so indexes can be built and indexing initiated. Delete the labels and the email from Gmail (you will probably need a gmail filter).

I won't do it all at once. I will move the messages in batches to local folders. I'm just wondering if mapping a gmail mail size of 30 GB is possible because what if that mail size causes the app to crash?