What is the smallest valid transaction that can be included in a block?

The minimum serializable transaction is 10 bytes.

• Version (4 bytes)
• Number of inputs (1 byte) (zero input)
• Number of outputs (1 byte) (zero output)
• Lock time (4 bytes)

However, since a transaction must have at least one input and one output, such a transaction is invalid. A minimum valid transaction requires 60 bytes.

• Version (4 bytes)
• Number of inputs (1 byte)
• Input #0:
  • Previous TXID (32 bytes)
  • prevout index (4 bytes)
  • scriptSig size (1 byte) (scriptSig itself is empty)
  • Sequence (4 bytes)
• Number of outputs (1 byte)
• Output #0:
  • Amount (8 bytes)
  • scriptPubKey size (1 byte) (scriptPubKey itself is empty)
• Lock time (4 bytes)

Such transactions should use non-segwit outputs that allow empty scriptSigs (such as bare scripts). OP_TRUE.