You could try looking at existing scripts. Scenes can be anything from half a page to twenty-something, from what I've seen studying English.
The only guideline I can think of giving is to ask one question: does it do what you need it to do? If yes, then it's fine - My first scene is just over a page long. I wanted it to set the atmosphere and plotlines of the play, and I feel it does - therefore, it's long enough. If you try to flesh out, people will notice - just like in essay-writing, excess information is noticeable.
Hope this helps - if not, I hope someone better experienced replies ^^ hehe
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