Display dimensions of new poly, Constraints not displaying?

[DeanChamberlain]

Hi, my work (uni) has just stopped allowing ArcMap, so I've moved across to Arc Pro.

I've imported an .mxd I was using in ArcMap to ArcPro. I do have a backup .mpk if needed, but from memory it loses all the colours etc, which with 20+ layers is annoying. 

I'm trying to draw a 50 x 20m polygon, adding to an existing layer. The dimensions are important. When I turn on the Constraints button, highlight distance as a constraint, it still doesn't display anything like it should. I've tried restarting Arc, and the computer, but nothing. You can see from the pic that the Constraints is on, but the polygon I'm drawing isn't showing anything. 

What am I missing here? Is this an artifact of bringing it across as an .mxd file?

Any suggestions appreciated. 

Thanks

[DeanChamberlain]

Apologies, I can't find the edit button.

Loading the mpk file into a new map doesn't make a difference, and other layers display the constraints, so it's not across the map, it's this layer. 

[Robert_LeClair]

To create a polygon feature constrained by a distance, you can follow the directions in the help documentation here.  The Edit tab is at the top of the ribbon and is where one "starts" an edit session.