Buy some LED lights and run them through the coop and run on a timer so they run from 6 AM to 7 PM or thereabouts.I'm in Minnesota, close to Minneapolis. Goal wouldn't be to keep it really warm, just keep the chill down. Was thinking of the tile warmer fabric, laying tile over it then cover in 3" of sand. The sand would become a heat sink and keep it somewhat warm.
Only reason I thought to insulate it was I heard egg production can go down with cold. I do a large amount of baking between Thanksgiving and Christmas that would be great to do with fresh eggs.
Its the light, not the cold that does it.
Assuming you're planning on buying 25 chicks, that coop size is fine since you're likely to experience some mortality or roosters (or maybe a hen) that needs to be rehomed, whether you rehome it to the frying pan or another farm is another story.