To be an "absolute size"? The size of the logo is the size that you created it in. I would not recommend modifying that via some other method - create it in the size you want it to be.
Ah. We have actually discussed this before. Try a search. Or, do a google search. You will need to actually modify your templates and add in an image manually and then write some CSS that will change it's size based on the browser site. (I can't remember if there was javascript in there too.)