What are the minimum sizes for transactions and blocks in Bitcoin? I have gone through many research articles but couldn't find out what the minimum sizes of a transaction and a block are.
Is there any reference where I can find an answer to my question?
There is no minimum size restraint on blocks and transactions. However, due to the nature of blocks and transactions, there is a practical minimum.

The smallest transaction I can think of is 61 bytes. It is a transaction that spends an OP_TRUE anyonecanspend output and creates 1 OP_TRUE anyonecanspend output. The smallest block I can think of is 146 bytes. This is the 61 byte small transaction + 4 bytes for block height in that transaction + 80 byte block header + 1 byte for transaction count.