I have one shed, and an old barn/shed that I use. My newest shed is 10x20, and I wish I would have gone bigger to just consolidate both into one...
in the 10x20, I have 3 double wide rat racks and a mouse rack on one wall. a mouse rack and grow out rat rack on the other, and a hamster rack on the back wall... Never enough space!

As for insulation, I just used fiberglass rolled with damaged paneling that I bought at one of those discount home improvement places. The paneling is water damaged, so some is discolored, but it was only $3 a sheet....and it's a rat shed

For a/c, go big! my 10x20 has a 12k unit, and on 105 degree days, it struggles.. Don't save money here, it will hurt you in the end. and, buy an old style unit with knobs, not digital, in case the power goes out, the older style will come back on unlike the newer digital style. Ask me how I know this. Lost 70 colonies of mice and 70% of the rats over this summer....

Heat, oil filled radiator is the way to go, they take a while to warm up, but, if the power does go out, they keep the heat up for a couple of hours til you can get something else going if the power goes out.

I learned the hard way this past summer, my worse accident in over 20 years of breeding. Invest in a power fail/freeze alarm if the shed is away from home too... $200 is alot cheaper than what I had to do!