I don't understand your question.
In all the years I've used TM, I have never once had it tell me how big the backup HDD had to be.
At what stage and where, is TM telling you this?
Unless this is the estimated size of a full backup info in TM-Preferences-Options, if that is the case then you need to cull out (exclude) everything except your start-up drive.
You have a Mac Pro?
How many hard drives do you have internal to your Mac Pro, and what is the TOTAL of all that space?
Maybe you only need to tell Time Machine to exclude some of your drives, or exclude everything else except your boot drive?
Otherwise, the first backup would be everything that your Mac Pro system uses for space.