I understand that the starting dimensions must be 11"x19"x15". Can the robot exceed those dimensions during the running of the competition? For example, ours folds up to be within the constraints at the start, but can lift up over 15" to complete some tasks.
Welcome to the vexforum.
For any question about the legality of something, we must turn to the game manual (or the Official Q&A). As you read the game manual (https://content.vexrobotics.com/docs/2022-2023/viqc-slapshot/VIQC-Slapshot-GameManual-2.1.pdf), keep in mind that the game manual tells you things that you must do, and things that you are prohibited from doing. it can’t tell you everything you might do, so if the game manual does not prohibit an action, then it is generally legal. The game manual does not generally discuss any particular game strategy or mechanisms you might use.
In regards to your question, you need to read the manual specifically for “expansion” found in rule G-13 and the note to G13.