Accordingly to RRoP section 14:
POT-LIMIT RULES
[...] The maximum amount a player can raise is the amount in the pot after the call is made.
I had a little discussion about the maximum preflop wager allowed.
Let's say SB is 50 and BB 100. It's clear that a player in any position can put a maximum bet of 350 (bet 100 + raise 250), being the first to bet.
But what about the SB, if all previous players folded?
And how much the BB can bet, if the SB was the only player to make action (call)?
Thank you in advance.
Elder to answer your question, the answer is 300 as Oddvark explained. Both the SB and the BB can only make it 300.
Stuart, I'm

at your link. I'm surprised at the answer. It is not a 'grey area' or 'undefined'. In the video, the questioner asks if the Blinds are 5,000-10,000 but the BB only has 5,000 in chips, how much is maximum bet to come in? Thankfully, Mr. Frezer didn't let us down by saying the BB is the BB and it's 10,000.
In flop games with blinds, the structured BB amount is always the minimum amount to call and is treated as a full bet even if the BB has less chips than the minimum bet required. Thus, the full amount of the BB is considered complete when calculating the pot for purposes of raising the pot.
Now that's tournament rules Elder and true Pot-Limit.
As some have alluded to, most cash games, particularly American cash games (Europeans tend to like true pot-limit), the SB is treated as being equal to the BB when calculating the pot. If there is a straddle, all other blinds are treated as being equal to the straddle.
So, if blinds are 25-50 (American cash games) the max opening bet is 200. SB 50 + BB 50 + 1st Call 50 + Raise 150 more for a total bet of 200. If there's a straddle, usually it's double the BB (but could be more), then all other blinds are treated as equal to the amount of the straddle. So if the blinds are 25-50 and there's a straddle for 100, the opener (who is not the SB or BB) can make it 500 (SB 100 + BB 100 + Straddle 100 + 1st call 100 + 400 more).