Who determines the block's size?

[BlackHatCoiner]

I've seen "block size limit is 1MB", other times I've seen "block size limit is 2MBs" and others "block size limit is 1.7MBs". Who determines that? The developers of bitcoin core? Why do they change it frequently? (if they change it and I'm not wrong)

[Ucy]

In my opinion, it should be determined mostly by their merits especially in regards to Bitcoin principles of decentralization, transparency, privacy/anonymity, immutablity, censorship resistant, security, permissionless/trustless, deflation, (efficiency too), etc. If a proposed blocksize will negatively affect those principles, it probably shouldn't be allowed, but if it enhances/improves or doesn't negatively affect the principles, it should be accepted.
 There should be ways to allow the community vote based on those merits. Current voting system not very effective in my opinion

[hatshepsut93]

Block size limit used to be 1 MB. Then with the SegWit softwork Bitcoin started using a new system called block weight. Under it a block is limited to 4 MB of block weight, and each byte of transaction data is counted as 4 bytes, except for witness data which is counted as one byte. This means that full blocks with weight of 4 MB can have very different actual size, depending on how much transactions are SegWit transactions. Theoretically it can get close to 4 MB, on practice the blocks are much smaller.

If you want to check it yourself, go to any block explorer and look at block information, you will notice that all blocks have the weight of 4 MB, meaning they have hit the limit, but their size is between 1 and 2 MB.

[o_e_l_e_o]

Who determines that?

The community and the developers. If you decide you want to use larger blocks, there are plenty of altcoins you can pick from which do so.

Why do they change it frequently?

They don't. Block size has only been changed once, from a limit of 1 MB to an effective limit of 4 MB, as hatshepsut93 has explained above.

If you want to check it yourself, go to any block explorer and look at block information, you will notice that all blocks have the weight of 4 MB, meaning they have hit the limit, but their size is between 1 and 2 MB.

That's not quite accurate. First, block weight is measured in weight units, not in MB. Secondly, not all blocks have a maximum weight. It is certainly the case at the moment, because the mempool is full and therefore every block is full of transactions, but if you look back a week or so to when blocks weren't full, then you will find plenty of blocks with a weight far below the 4 million limit. For example, block 629169 had a weight of only 577 kWU.