It is, of course, very early, as other posters have said. But I understand the value of having a few “proof of concept” schools to try on for size.
The one that comes to mind for your criteria is the University of Miami. It’s on the larger end of mid-sized (12K undergrads) but it’s hard to find small schools with robust marching band programs, so this is probably a good compromise between small and huge, as many big-football + marching band schools are. UMiami has a great, but reasonably inclusive, marching band program https://bandofthehour.miami.edu/ as well as great music in general through its Frost School of Music. The School of Communications has majors in Cinematic Arts (multiple tracks) and Interactive Media, and the Art department in Arts & Sciences has Graphic Design, Illustration, & Multimedia major. Admissions are competitive, with a 27% acceptance rate, but median stats aren’t sky-high, so it’s likely a realistic target if your son keeps her grades up as he has been. As such, it’s a good level of school to develop a fondness for at this stage of the game - not a highly-rejective “Hail Mary” reach school, but one that does require the kind of strong credentials that are attainable if she keeps up the way he’s going.
What you’ll need to figure out over time is whether your son aspires to programs that are ultra-competitive in one way or another. There are the schools that are just hard to get into in general. (Example: Notre Dame - 9000 undergrads, serious football & marching band, and has both visual communication design and film/television/theater majors) There are the schools where film in particular is extra-competitive and would require a strong portfolio to even apply (Example: U of Southern CA, which is also not an easy admit in general… 20K undergrads, marching band, multiple design and film-related majors.) There are schools where, as mentioned above, getting a spot in the marching band isn’t guaranteed. (I knew one kid through CC who chose his school for the band and then found out he didn’t get a spot. Happily, he ultimately got a spot off the band’s waitlist after all, and ended up having a great experience, but there was a period of big heartbreak before that came through.)
The other consideration, if he wants one of these creative majors, is whether he wants an intense/immersive BFA program or a BA program that operates like other “normal” academic majors. The former can make it harder to make other time-intensive commitments like marching band… but if she’s super-serious about film or design, a BFA could be the way to go.
So, lots to think about.
One more thought would be Belmont University in Nashville. (Caveat: this is a Christian school, and opinions differ on how comfortable an environment it is for non-Christian or less-religious students - YMMV.) Belmont is very highly regarded in music and other creative fields, and it has both film and graphic design programs. It has only about 5500 undergrads and no football, but nearby Vanderbilt University has a marching band that welcomes participants from Belmont and several other area schools. Vanderbilt Spirit of Gold Marching Band | Vanderbilt Athletic Bands | Vanderbilt University So that’s one way to attend a small, artsy school and still get the football/marching band experience!
Good luck with figuring things out over the next few years!