To indicate future "will sit" I would use leüülni Okay?
"Melyik székre ülsz le?" would be just as correct. This verb in present tense can indicate future with or without the prefix as well.
Melyik székre fogsz leülni? Melyik széken fogsz ülni? these are also good for future tense.
Would széken also work for on the chair?
Zsuzsi is correct. Some would also use "majd" to indicate some undetermined time in the future, as in "Majd melyik székre ülsz?"