Can I lock the wheels to keep it stationary while I'm working?

Locking mechanism specs are not detailed in the product data — inspect the base when it arrives and test the wheels under load before committing to any workspace layout.

Are the shelves adjustable, or are they fixed at three heights?

Shelf configuration is not specified — treat them as fixed positions until you confirm otherwise during unboxing. You may have limited adjustment depending on the design.

What's the footprint, and will this fit in a standard garage bay?

Dimensions are not provided in the listing — measure your workspace before ordering, especially if storage is tight or you need to maneuver around other equipment.