transactions – Is there a set size {that a} signature will be?

Throughout the script language there are two forms of directions:

Knowledge pushes are encoded by a single byte n (with worth between 0 and 75 inclusive), adopted by n bytes (the information to be pushed). Its impact throughout execution is pushing that n-byte worth onto the stack.

Some other byte worth (so between 76 and 255 inclusive) denotes an opcode. Most opcodes are only a single byte, apart from OP_PUSHDATA1, OP_PUSHDATA2, and OP_PUSHDATA4, that are used for pushing bigger knowledge components than 75 bytes, however are nonetheless thought-about opcodes.

Within the notation you cite above, <X> is a shorthand for “knowledge push of X”. Within the precise byte encoding, which means it’s prefixed by its size. So particularly, OP_0 <A sig> <B sig> OP_2 <A pubkey> <B pubkey> <C pubkey> OP_3, e.g. when A sig is 71 bytes and B sig is 72 bytes (and the pubkeys are all 33 bytes):

• 0x00 (OP_0)
• 0x47 (knowledge push of size 71 follows)
• A sig‘s 71 bytes
• 0x48 (knowledge push of size 72 follows)
• B sig‘s 72 bytes
• 0x52 (OP_2)
• 0x21 (knowledge push of size 33 follows)
• A pubkey‘s 33 bytes
• 0x21 (knowledge push of size 33 follows)
• B pubkey‘s 33 bytes
• 0x21 (knowledge push of size 33 follows)
• C pubkey‘s 33 bytes
• 0x53 (OP_3)

Notice that within the instance you are referring to, this complete sequence is not really realized as a single script. The general public keys would sometimes be within the scriptPubKey or redeemScript (and possibly adopted by OP_CHECKMULTISIG), whereas the signatures can be within the scriptSig. It is solely throughout spending that each get executed so as, making it equal to having this single script.

Notice additionally that the script language itself doesn’t know or care in regards to the distinction between public keys and signatures – they’re simply knowledge components on a stack. It is the OP_CHECKMULTISIG opcode that follows which will likely be decoding the information on the stack (and it is guided by the two and three pushed by OP_2 and OP_3 to know this can be a 2-of-3 multisig, thus decoding the opposite components on the stack as 3 public keys and a couple of signatures).

Now to reply the query in your title: since BIP66 took impact in 2015, ECDSA signatures will be at most 73 bytes.