I've never done a workshop at Shrewsbury but've encountered most of the options at various UK festivals, doing both writing and arranging. The smallest number was one guy who didin't actually play, sing, or write - the biggest was about 60, in which case you really have little option but to do a lecture/demo (and bear in mind that this is what most workshop leaders do, so many - even most - of your audience may be expecting/wanting this).
My infamous songwriting booklet was written as a direct response to a workshop at Broadstairs where I had four people - one who had never written a song and didn't ever envisage doing so, one who was brilliant with words but almost tone deaf, one who was a great musician but had poor language skills, and one who was a far better songwriter than me!
It's really only practical to do hands-on writing if you have two or more linked workshops, so people can go away and work quietly by themselves then return with something for the group to consider.
Don't be afraid of commenting on other people's work. There will always be something nice to say, and you'll know how hard to go in with the shears by their reaction. The important thing is to leave them believing they can do it as well as you - because they probably can.
A week of daily workshops at Sidmouth (in the middle of the night, though - at 9.30 a.m)! produced 5 good songs, one I even recorded on my next album, but you'll find you won't get much done in terms of writing in an hour and a half - though you can pass on some good tips (and some of the attendees will do too if you let them). This is why I wrote the book - because I could see that I could never satisfy all the questions in the time and I didn't want people to go away Empty Handed. (Unless they were going to write a song as good as that natch).
Arranging is easier - if more restricted.
Tom and I have one, but even that varies according to what people want. Sometimes it's about putting chords to tunes or song melodies, sometimes it's about editing lyrics, sometimes it's about duo/band arrangements - and how instruments work together, or about presenting music on stage (how to add drama etc).
The best one about arranging was with Johnny Dyer and Vicky Swan AND Chris Parkinson and Maggie Boyle at Sidmouth. They all knew some of my songs, so we were able to play a couple of things quite well from the off, and discuss our individual approaches. Then we had a go at arranging an unaccompanied song of Maggie's that the rest of us had never heard. It was a mess, of course, but rather a lovely mess, and I hope enthralling for all concerned. That was a biggish crowd (obviously) so there wasn't much interaction, but Tom and I used to do schools work which was 100% hands on.
Flexibility is the key.
Good luck :-)