visio basics question - I want to use it to show where plants are in my yard

I've been using visio to document where we are putting plants in our yard, the type of plant, when we bought it, etc.

but it's just relative positions - I took a visio page and started putting plant icons on it.

I want to make it more accurate, with a scale 1" = x', etc.  but don't know how to do that.

How to draw a box that is x' x y', put a bush x' from this edge, etc.

any beginner advice?

It's a bit OCD / compulsive, but figure it'll help me learn visio for real work : )