To figure out the sizes of things, use these two tricks for now:
A simple solution to ge the EXACT dimensions is thus:
A) open paint program
B) make a 1x1 pixel image
C) Color it some obnoxious color (orange is good here)
D) add the alpha channel, make it opaque
E) Save as 32 bit TGA
F) Use it in place of whatever thing you want to get the size.
G) Open LOTRO
H) Change skin
I) Look for change.
J) Take screenshot
K) Open screenshot in paint program
L) Measure the orage area of whatever was replaced.
Just a little information for other skinners who haven't figured this out yet:
If you create a SkinDefinition.xml file specifying a non-existant image file the client will load a grid pattern; allowing you to use the above method for getting the size without the need for creating an image file.
You can see the results of you image changes without reloading the client;
1. Load up the client and login to the game.
2. Load up your skin, this is how it is now.
3. Change back to the default & alt+tab into your image editor.
4. Make your changes & save them
5. alt+tab back in to the game & change back to your skin, you should see the update image.
P.S. the bottom toolbar is 1074 x 85 pixels.
Both taken from the Skinning the LotRO UI thread in Turbine's Beta UI Forum.