This may seem like simple situation, but it's really quite important because these are essential questions of betting... With minor modifications of Ricky's text
Blinds .50/1.00
folds around to SB throws in 1x5.00 and 1x1.00. Doesn't touch SB of .50.
a) raise to 6.00 OR
b) raise to 6.50
The basic rules of no-limit betting include: 1) Raise must be at least the size of the largest bet or raise of the current round (TDA Rule 43-A "Raise Amounts"; 2) All chip denominations from largest to smallest can potentially be bet, as long as rule #1 is satisfied.
With that in mind.... SB puts out .50, BB puts out 1.00. The next player who wants to raise must raise at least the size of the largest bet (the BB at 1.00), for a total minimum raise of 2.00.
In this case the SB put out 6.00 extra on top of the .50 already in front of him. He didn't disturb the .50, leaving the clear impression he wants to bet the entire 6.50. 6.50 is a legitimate bet because it's in excess of 2.00 and all chip denominations in play are valid betting objects. If some denominations aren't valid for betting, they shouldn't be on the table.
So total bet of 6.50. See it another way... let's say the 6.00 were all the chips remaining in the guys stack and he pushes it all out... he's all-in for 6.50.
This raises the related question addressed by TDA Rule 47: "Previous Bet Chips Not Pulled In". The SB already had a bet in front of him (.50). How do we treat the many possibilities if he adds chips to that previous bet, but a) disturbs the prior bet; b) disturbs half the prior bet; c) just puts a single new chip out (a max raise, or a call?); d) puts a single new chip out but combines it w/ the prior bet and tosses it all forward ?? Etc. etc. For the last two Summits the TDA delegates have debated on codifying all the possibilities. At the end of the day it proved difficult to find agreement on many of the possible actions. Hence in 2015 the Association ratified the 2013 language, putting the responsibility on the player to clarify his action, and recommending declaration prior to putting out new chips on a previous bet that has not been pulled in to the pot. In 2015 the Association also adopted 47-B, that pulling back a prior bet when facing a raise binds you to at least making a call... you can't toss the bet back in and fold.
Lastly, let's say the SB had first pulled back the .50 SB, then tossed out the 1.00 and 5.00 chips... I would have no problem ruling that a total bet of 6.00. But because he didn't disturb the .50 SB, he's betting everything.
Many thanks to Ricky for presenting a great case.