I know that I can make sprites bigger (with sspr()) but is there any way to display map tiles at, say, 2x size?

nothing built-in but look at tline in manual

for map specifically, you can use this: https://www.lexaloffle.com/bbs/?tid=38931

to zoom everything on screen, you can use video remapping (see https://www.lexaloffle.com/bbs/?tid=45538) to map spritesheet addresses to screen data, then draw with sspr!

Thanks, @merwok. I'll look into that. It looks like just what I want, so thank you!