I've just bought a home sandblasting unit and wondering what grit size I should be
Using as I don't want to warp and destroy panels

I vote crushed glass, nothing else.

In blasting 'hardness/impact strength' indicated rating terms, it rates around 6, and the rated strength of our old Mazda's thin metal is about 6.5 i believe, and im sure would best suit your needs.
Cam up here at Imperial Blasting is a machine, and does nothing but cars and parts, and this is what he uses on old Mazda's and simular vehicles.

Im NO expext, so await more advice.

Crushed glass.

I run a blasting company for a living and blast many cars and I normally use a combination of grits to get the job done.
Walnut shell on flat panels (if needed) and fine glass on everything else.

Just remember to work one layer of paint at a time and don't stop on one spot for to long.
Hold the nozzle on an angle to the panel and not straight on.
Blast at around 60-80psi.
Anything that pops in and out like a tin can won't handle any hard abrasive blasting you will warp it.

How big is your compressor?
You need decent air for blasting.

Fine glass can be a pain in the but to get to it blast you need very dry air.

Seriously I would take it to a reputable blaster and get it done properly.
There is a bit of a knack to blasting auto steel.

One other thing is don't be stingy on using a good primer.
Use a good epoxy primer.

Thanks guys I've just ordered a 25 kg bag of fine grit crushed glass I'm not goin to be using it for entire panels just little bits and pieces here an and there and cleaning up rusty metal
Yep I've got me some good quality epoxy primer already cheers for the advise

If your only doing bits and pieces I would use a 150mesh garnet so you can at least reuse it a few times.
The glass turns to dust after the first use.

Wear good protection !!