It's interestingly different from the VIC-II;
Not at least, because they represent complete different concepts. ANTIC is just a half of the graphics system, G/CTIA being the other.
The VIC-II is fixed in its capabilities, with its DMA being tied to simple display fetches. ANTIC's DMA is way more fexible and programable (Keyword here is Display List programing)
in particular, there isn't the sheer bulk of sprite circuitry,
These are within the 32 Registers of the G/CTIA, which in turn gets feed by DMA access from memory, controlled by the ANTIC.
presumably because it doesn't have as much in the way of sprites.
It doen't need them. Where the VIC-II needs dedicated RAM to store limited sprites, the ANTIC/GTIA pair uses the whole CPU memory to produce unlimited sized (in height at least) sprites.
How much on chip memory does ANTIC have?
Dependign on how one counts 48 to more than 70, as the registers also data. The 'huge' 48 byte block is only the line buffer.
What is it used for? Sprite data? Or does it have other stuff like the way the VIC-II has to store forty bytes of characters?
Somewhat. For text display it's much like with the VIC, but it also works for grapic modes. Depending on the mode selected for that line 8 to 48 bytes are loaded with character (name) or grapics data. And kept for the following lines.