Here is how they list it:
You must purchase:
# a XXXXX and/or
# a XXXXX OR a 22 oz drink.
You MUST pay for your items with CASH.
It also says:
*You may purchase a XXXXX or larger drink size, but you will only be reimbursed up to the $5.75 maximum. You may purchase a XXXX or other XXXX item INSTEAD of the XXXX if needed.
On one hand it looks like that you can get a larger size drink, however, being that it uses the word "INSTEAD" later on, I don't know if they will reimburse up to the total if you buy two.