Hey greasontim, its a common problem but very easy to solve.
I did this with a scaling value of 0.50. If yours is different you must experiment with the sizes until you are completely happy with the look of your barracks.
1) First open up your object editor and go to the barracks that you are using in game.
2) Next go to Art - Ground Texture.
Definiton - The Art - Ground Texture is what the art below the building will look like.
I found that with the scaling value of 0.50, the Orc Shipyard went well with my barracks but experiment with yours until you are happy.
3) Then go to Art - Selection Scale.
Definition - The Art - Selelction Scale is the green ring around your unit or building when you select it.
I found that with the selection scale of 3.00, the barracks with the scaling value of 0.50 perfectly fit it. Experiment the selection scale to fit yours.
4) Then go to Pathing - Pathing Map.
Definition - The Pathing - Pathing Map is the size of your building when built and the area of your building when its built.
I found that with my barracks having the 0.50 scaling value, the circle of power pathing map really suit my barracks. Experiment to find out yours.
Thats it your done! If you need any more help PM.
Happy Mapping.