Do you think the previous owners installed a sink drain in the shower?! (moved in two weeks ago)

The shower does seem to be on the smaller side.

I REALLY, REALLY, REALLY want to replace this drain, because I cannot remove the stopper to clean it. How do I determine if a normal shower drain will fit?

What size drain is that? There is nothing that shows scale to anything there. It does look like a vanity sink drain, but that would be really oddball, and terribly incorrect. Also, WHY would you want a drain with a stopper in a shower to begin with? They are not deep enough to serve as a tub, and there is no overflow.

Sorry...not being trolled. I simply don't know a lot about plumbing, hence my questions.

Looks like a bath tub. Kelly, posting pictures helps. You maybe using the tub as a shower, but you have a bath tub. Stax asked about an overflow….your pictures show one.

There are many websites with removal instructions. I kept turning my tub stopper until it released.

They also chose to ignore and not answer this question: "Do you mean to say a ”tub drain”? "

That would have explained everything.