Right, balancing that length is definitely important haha. I mostly gauge by like... premium comics, I guess. I'll never have an episode longer than those (and on average, quite a bit shorter) so good to go!
But yeah, for my first series that's up on Tapas, I ended up going with a 7 page first episode, which got people all the way through the first scene. I felt like that was a good litmus test for new readers to make an informed decision about whether they wanted to read on or not. Afterwards I averaged 1-2 pages per update. I feel like it would have been a really slow start if that first bit took almost 2 months to get through.
With my new project I've switched over to vertical format with longer episodes by default so I didn't see the need to do a longer first ep. I did go back and make a new episode 1 ahead of my launch later this month though
I built up 6 episodes of buffer and thought "ok! Am I good to start uploading?... Nope! Episode 1 is NOT a good introduction" lol