blocksize – regarding the size of a transaction in a block

What about the second? This can be 61 bytes long?

Yes.

In addition, each transaction has a counter?

No. There is a global counter in the block just after the block header. This counter is the number of transactions included in the block. It is not part of any transaction and its size does not affect the transaction size of the block (compared to when this transaction was not confirmed and in mempool).

The transactions come after this counter which has just been listed one after the other without any additional information separating them (the transactions follow a specific format, so it is trivial to know when we finish and when the next one begins).