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This question already has an answer here:

If I know the m of n values of the multisig, the # of inputs (all of which are from the multisig and require m signatures) and outputs and the redeem script, how can I determine the size of the transaction after m signatures?

marked as duplicate by JBaczuk, Andrew Chow Dec 24 '18 at 15:38

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • @JBaczuk there doesn't seem to be an accepted answer there, nor does the existing answers fully provide an answer. – tipu Dec 22 '18 at 0:39
  • @tipu to me it seems that the voted answer in the other question fits and answers your question. It assumes that p2sh-multisig is being redeemed, all pubkeys are compressed (33 bytes per pubkey) and that the signatures produced are 72 bytes each. If you're asking about segwit specifically (the calculation will be a bit different in that case), then please edit your question. Otherwise I agree that it's a duplicate. – arubi Dec 22 '18 at 9:05