Do you have a "standard" one gallon jug? By that I mean the kind that, say, Carlo Rossi wine came in years ago. I have several one-gallon jugs and a 6.5 stopper fits them well and a 7 can be squeezed in with some effort. The diameter of the opening is 1 and 1/16". You are saying that a number 7 is "too small," so I am guessing that you have a unique jug.
This might help you. Looks like a #8 would be just the ticket.
Stopper Size | Bottom Diameter (Inches) | Top Diameter (Inches) | Avg. Diameter (Inches) |
#2 | 5/8 | 3/4 | 11/16 |
#3 | 11/16 | 7/8 | 13/16 |
#5.5 | 15/16 | 1-1/16 | 1 |
#6 | 7/8 | 1-1/4 | 1-1/8 |
#6.5 | 1 | 1-1/2 | 1-1/4 |
#7 | 1-3/16 | 1-7/16 | 1-5/16 |
#7.5 | 1-1/4 | 1-1/2 | 1-3/8 |
#8 | 1-5/16 | 1-9/16 | 1-7/16 |
#8.5 | 1-7/16 | 1-11/16 | 1-9/16 |
#9.5 | 1-1/2 | 1-3/4 | 1-5/8 |
#10 | 1-11/16 | 1-15/16 | 1-13/16 |
#10.5 | 1-13/16 | 2-1/16 | 1-15/16 |
#11 | 1-15/16 | 2-3/16 | 2-1/16 |
#11.5 | 2 | 2-1/2 | 2-1/4 |