I know that till is used as a preposition for time . But if I say, " I want to trim my hair till the shoulders" would that be correct? If not then which preposition would be more appropriate for the given sentence?
As a conjunction or preposition 'till or until / 'til' are used only to talk about a time point. To talk about distance, as far as, or up to can be used.
Until as a preposition means ‘up to (the time that)’:
We played chess until midnight. (up to midnight)
You can trim your hair over/above/up to the shoulders, NOT till the shoulders.
We use until as a subordinating conjunction to connect an action or an event to a point in time:
Let’s wait here till the rain stops. (till + subordinate clause)