The sauna in the game is so big and clunky, is there any CC for a smaller one (like with only one "bench" instead of a whole U-shaped contraption)? The existing one hardly fits anywhere, I even tried sizing it down but it didn't work. I just want my sims to have a nice little at-home sauna that isn't absolutely massive, we're not millionaires in a mansion here lol

Did some reasonably extensive searching for things like 'sims ts4 smaller sauna' and 'sims ts4 sauna mesh edit' and some searching on specific CC hosting and finds sites, couldn't really find anything. New meshes the same size, yes. Smaller items that are deco only, yes. But not any actual functional smaller saunas.

Making a functional smaller sauna would require someone to be able to both make the smaller mesh (or get permission to use someone else's or get someone to make it to their specs, etc.) and (perhaps more importantly) create a custom tuning for the new mesh to correctly account for the new dimensions and associated capabilities of the smaller object -- it'll need changes to the rig/slots that determine where Sim bodies go when they interact with it and animations are played, changes to the tuning that determines how many Sims can interact with it at once, ideally it'll have Woohoo VFX that aren't 3x3 tiles like the steam cloud in the existing sauna, etc. Lots of ways one could, potentially, make such a thing -- it depends on how one wants it to work.

You might have an easier time just putting something in your Sims' house that fills whatever function(s) you wanted the sauna to fill for them, but takes up less space. Otherwise, unless someone else turns up something I couldn't find, you could always try posting in a place where requests are permitted (like the relevant section of the S4S forum) or looking for someone who does both meshing and tuning and takes requests or commissions.