You have to set up the UI scale first (through blizzard interface)
Most addons have a locking/unlocking feature as you prob know, move the addons where you want and scale them up/down to whatever size you want.
By default if you download a UI pack for example. The person's UI you downloaded is custom to his monitor and resolution and set up, so you would need to move and scale the addons to your preference.
It wouldn't be much a custom UI or whatever if you get someone to give you their set up.
I mean, even if someone gave you their set up for 1920x1200 resolution, you might not like how big they set their cast bars or where they put their action bars...
