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I'm looking to convert some other things to Chaos Spawn models but don't have any real ones to compare with.

Firstly what are the approximate dimensions of the Chaos Spawn models? (I'm aware there are a few variants)
Are they on a standard 25mm base?
Any tips for other models on which to base the conversions? Nids come to mind perhaps. Some Daemon bits?

not sure but i think theyre on 40mm bases.

Yes, it's 40mm bases
The Warriors of Chaos Forsaken kit looks like it has some cool mutation bitz in it. Or the CSM possessed kit.
Just from looking at the pictures on the website, the spawn models appear to have a similar size to SM Terminators (about as high as the base's diameter which in both cases is 40mm)

Yes looking at pics a 40mm base looks to be right. Any ideas how tall/wide they are? Obviously they are a bit bigger than normal infantry sized models, perhaps not as big as a dreadnaught/helbrute though.

I just updated my previous post.
Adding to that, I don't think you should worry too much about width, since arms and tentacles don't count for the purpose of LOS as far as I know.
If you check the WHFB spawn entry it tells you that the base is 40mm (both the square and the round one)

Spawn are about half again as tall as a space marine, not including the various appendages. This doesn't account for things like worm spawn, or giraffe spawn, or whatever else that they might mutate into. Seriously, as long as they're on a 40mm base you can use nearly anything as a chaos spawn.

As far as models to convert, Reaper minis is a cheap source of assorted monsters and monster bits. The spawn kit itself, while pretty expensive, comes with enough bits to kit out the two that come with it and have enough tentacles and flailing organs left over for easily 3 more spawn.

I use some of my old 'Nid bits, a good helping of green stuff, and a few shots of Kraken rum. Let the imagination run wild.

Spawn are arguably the most open to interpretation of any model you'll field, so have fun with it.

Carry on.