what is exact meaning of some field of response of `getBlock` rpc api
size refers to the size of the block, which is 80 bytes for the header + sum(tx_sizes). As pointed out in the comments, this includes the segwit data and is meant to match the actual, on disk size of the block.

bits refers to nBits, which encodes the target difficulty for the block. You can see an explanation of the encoding scheme here.

chainwork represents the sum total of the expected hashing operations required to product the chain up to this block - this means that you cannot use it for calculating the hash power for the current block, as it is the sum of all the hashing required to produce every block from the genesis block up until the current block.

In fact, you can never get a precise measurement of the active hash power based on a single block. However, there are ways to calculate the average/expected hash power on the network at a given point in time, as described in this question.