First, I assume you refer to the official US flag that is flown 24/7 over the White House, not flags that are merely hoisted for a moment for commemorative purposes, then hauled down. I've been looking for an hour; so far nothing FACTUAL about flag sizes at the White House; a lot of "misdirection" answers that reply with US Capitol flag information (more on that in a moment). Unless they read carefully, unwary readers (like me) can be fooled into thinking they've found your answer.
So at present, I have not found an answer. I'll keep looking!
That being said, let me share some collateral info with you. People can buy commemorative US flags that have actually flown (VERY briefly) over the Capitol building. These are ordered through the office of your US senators - a link is given below. I check 4-5 different senate offices; the standard sizes are:
3' X 5'
4' X 6'
5' X 8'
The bigger the flag, the more the $$$. Again, remember - these are US CAPITOL flags - NOT White house flags.
One more comment. US Naval ships and bases (and presumably other services as well) fly various sized US flags at different times. There are "regular size" flags for routine work days, larger "holiday" colors for weekends/holidays, and much smaller "storm colors" for foul weather (to save the cost of replacing bigger, more expensive flags). It may very well be that both the WH AND Capitol do something similar. But this is mere speculation on my part.
http://www.senate.gov/general/contact_information/senators_cfm.cfm
