Is the 撇 in 左 longer than the 撇 in 右?
As an upper-left part of a character, the length of the left-downward (撇) stroke in the shape 「」 is determined by the traditional* or calligraphic stroke order of the entire 「」 shape. Specifically, the left-downward stroke among characters where the stroke is written first appear to have a shorter length than those where it is written second.
Whether or not the left-downward stroke is written first or second in the shape 「」 depends on the object underneath it. Adapted from the Japanese SE answer I wrote here to the question 右 and 石 stroke order differences:

The order that「」is written depends on the object underneath it. If the component underneath it doesn't exceed the horizontal stroke of「」(e.g. 右、有、布), then write the left-downward stroke first; otherwise (e.g. 友、左) write the horizontal stroke first.
Examples (mined from jisho.org):
Bottom component narrower than horizontal stroke of 「」

Bottom component wider than horizontal stroke of 「」


I will emphasise that this description of the stroke length's relation to the stroke order only works within the framework of the calligraphic stroke order, and that no education standard† actually prescribes the calligraphic stroke order as a standard. Neither the Taiwanese (ROC) nor the Mainland Chinese (PRC) stroke order standards write 「右」 in a manner which is consistent with the calligraphic stroke order:

Credit: M4RC0, licensed under CC BY 3.0

*Not to be confused with Traditional Chinese. The traditional or calligraphic stroke order is the order used in Imperial China, and not attached to any education department of a modern government.
†The closest educational standard to the calligraphic stroke order is actually from Japan. For all of the the characters mined from jisho.org, the Japanese stroke order matches the calligraphic stroke order.