Depends on your bank roll and how many other bookies you’ve signed to to really. If you have a decent bank roll and a fair number of bookies, go larger. If you don’t go smaller.
I prefer larger as it means less work. Find one or two good bets where lads have higher odds than most other bookies, and it’s all done easily. But if you are going to put a $500 bonus on a 6/7/8-1 bet, you need some decent coin to put the counter/lay bet on.
Smaller will mean you’ll need to find more good bets, but you won’t need as much coin on the counter/lay bet.