For 3mm, my bases are 25mm x 12,5mm tor infantry and vehicles, 20mm x 20mm for weapons. This allows you to essily see what is what as a distance, as infantry is based wide and vehicle long.
I use posterboard for the basing. At these sizes it is too small to warp.
With regards to stirage, I have good news. 3mm is somlight it can store in pretty muc anything as long as you get a layer of foam over it. I buy discount plastic boxes and put the stuff in in layers, with a layer of foam between each. I make sure there’s a final layer of foam to pack it in tight. And that’s that.
We get slapped around, but we have a good time!