Welcome to the Guild PEB! As for the resizing of the base map... ready for it? You don't.

Shocking I know. Many, many before you have had exactly the same question. The thing to know is that CC works in "real world" units and in "infinite space". If you are working on an overland map, you can simple take the default of 1 map unit equals 1 "whatever" unit (typically a kilometer/mile) and go from there.

What is probably throwing you is the border. My advice there is just unfreeze the layers and delete all the entities. You can redraw or add a border later.

If you are working on a city or dungeon map, however, the units are 1:1. Where 1' = 1' on the scale of the map. Of course you can change this as needed as well.

I would strongly encourage you to go through the quick start manual that comes with CC and do all the tutorials. You will find them invaluable for getting up to speed very quickly. Once you have gone through all the tutorials you'll find that your questions will be much deeper and you'll have an easier time at understanding the answers that folks give you.