What is the biggest hard drive I can put in my iMac g3?

3.5"

https://everymac.com/systems/apple/imac/specs/imac_dv_se_500.html

Those models can not use drives larger than 128GB. They might work, but you can only address the first 128GB part of the larger capacity drives. "big drive" support was introduced in 2002.

Macintosh: Using 128 GB or Larger ATA Hard Drives - Apple Support

https://everymac.com/systems/apple/imac/faq/imac-g3-hard-drive-upgrade-replaceme nt-instructions.html

Much easier to use the FW400 external port, you can use larger drives, and it would probably be about as fast as the internal drive in that vintage machine. Don't use a USB drive, you only have the very slow USB 1 on that machine.

OS 9 info: Note, if you are booting into OS 9, the Mac OS 9 System Folder must be on a partition <200GB.

http://lowendmac.com/2014/how-big-a-drive-does-mac-os-9-support/

A couple of add-ons to Glen's excellent response:

1) If you can only find a larger drive like a 160GB, you can use Disk Utility to partition the drive so it has one partition under 128GB and the other left unformatted and blank. The working partition can't be set to exactly 128GB because the formatting and rounding of numbers involved will make that partition slightly too big to be usable. When setting up the working partition, best practice is to make it about 125GB. From experience we know that is a workable value that will always create a usable partition.

2) Bigger hard drives are newer than the cute little things Apple installed and they tend to run hotter. When I upgraded our iMac 2000DV 400mhz to a newer 60GB hard drive, we had a lot of heat problems that would shut down the computer. The G3 iMac is convection-cooled (no fans) and that is it greatest weakness. Anything you can rig that will force air up into the cooling vents on the bottom of the iMac case will help. So will a good eviction of any dust bunnies and other invaders that have found their way between the Faraday cage and the case bottom. You will have access to this area when you are replacing the drive.

1TB, most likely. Certainly not more than 2TBs.