You can see your staking options and choose your preferred level on the entry coupon of the relevant Colossus Fantasy Contest. For all Fantasy contests, the maximum (full unit) stake per line, which plays for the top prize, is $20 (or currency equivalent). There are then three fractional staking options: $10, $5 and $2 (or currency equivalent).
As you change your stake per line, the top prize you are playing for changes in proportion to your stake, as shown in the contest header.
