Since #11100, in init.cpp `-blockmaxsize` is translated to `-blockmaxweight` with the code:

```
        unsigned int max_size = gArgs.GetArg("-blockmaxsize", 0);
        if (gArgs.SoftSetArg("blockmaxweight", strprintf("%d", max_size * WITNESS_SCALE_FACTOR))) {
```
However SoftSetArg requires the argument to be specified with the leading dash (ie, as `SoftSetArg("-blockmaxweight")`) without the dash the value will not be looked up later (because the later lookups specify it with a dash).

Obvious fix is just to add the dash (`SoftSetArg("-blockmaxweight", ...)`), but if there haven't been any complaints, just removing the option entirely might be better, since current behaviour is the same as if the option is just removed.

I believe this applies to master, 0.16 and 0.15 (due to backport via #11592).

Test case to demonstrate:

```
#!/usr/bin/env python3
"""Test limiting maxblocksize"""

from test_framework.test_framework import BitcoinTestFramework
from test_framework.util import *

class MaxBlockSizeTest(BitcoinTestFramework):
    def set_test_params(self):
        self.num_nodes = 2
        self.extra_args = [["-blockmaxweight=40000"], ["-blockmaxsize=10000"]]

    def run_test(self):
        chain_height = self.nodes[0].getblockcount()
        assert_equal(chain_height, 200)
        self.nodes[0].generate(100)
        self.nodes[1].generate(100)
        self.nodes[0].generate(100) # unspendable

        blk = [None,None]
        for n in [0,1]:
            vsize = 0
            address = self.nodes[n].getnewaddress()
            txes = 0
            while vsize*4 < 84000:
                txid = self.nodes[n].sendtoaddress(address, 0.25)
                tx = self.nodes[n].getrawtransaction(txid, 1)
                vsize += tx["vsize"]
                txes += 1
            self.nodes[n].generate(1)
            blk[n] = self.nodes[n].getblock(self.nodes[n].getbestblockhash())
            self.sync_all()

        assert blk[0]["weight"] <= 40000, "blockmaxweight greater than 40000"
        assert blk[1]["size"] <= 10000 or blk[1]["weight"] <= 40000, "blockmaxsize is greater than 10000"

if __name__ == '__main__':
    MaxBlockSizeTest().main()
```

Cc @jnewbery (who picked this up via an assertion failure in #11862) @TheBlueMatt (who did the original PR)
{
    const auto defaultBaseParams = CreateBaseChainParams(CBaseChainParams::MAIN);
    const auto testnetBaseParams = CreateBaseChainParams(CBaseChainParams::TESTNET);
    const auto defaultChainParams = CreateChainParams(CBaseChainParams::MAIN);
    const auto testnetChainParams = CreateChainParams(CBaseChainParams::TESTNET);
    const bool showDebug = gArgs.GetBoolArg("-help-debug", false);
    
    
    std::string strUsage = HelpMessageGroup(_("Options:"));
    strUsage += HelpMessageOpt("-?", _("Print this help message and exit"));
    strUsage += HelpMessageOpt("-version", _("Print version and exit"));
    strUsage += HelpMessageOpt("-alertnotify=<cmd>", _("Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)"));
    strUsage +=HelpMessageOpt("-assumevalid=<hex>", strprintf(_("If this block is in the chain assume that it and its ancestors are valid and potentially skip their script verification (0 to verify all, default: %s, testnet: %s)"), defaultChainParams->GetConsensus().defaultAssumeValid.GetHex(), testnetChainParams->GetConsensus().defaultAssumeValid.GetHex()));
    strUsage += HelpMessageOpt("-blocknotify=<cmd>", _("Execute command when the best block changes (%s in cmd is replaced by block hash)"));
    strUsage += HelpMessageOpt("-blockreconstructionextratxn=<n>", strprintf(_("Extra transactions to keep in memory for compact block reconstructions (default: %u)"), DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN));
    if (showDebug)
        strUsage += HelpMessageOpt("-blocksonly", strprintf(_("Whether to operate in a blocks only mode (default: %u)"), DEFAULT_BLOCKSONLY));
    strUsage += HelpMessageOpt("-conf=<file>", strprintf(_("Specify configuration file. Relative paths will be prefixed by datadir location. (default: %s)"), BITCOIN_CONF_FILENAME));
    if (mode == HMM_BITCOIND)
    {
#if HAVE_DECL_DAEMON
        strUsage += HelpMessageOpt("-daemon", _("Run in the background as a daemon and accept commands"));
#endif
    }
    strUsage += HelpMessageOpt("-datadir=<dir>", _("Specify data directory"));
    if (showDebug) {
        strUsage += HelpMessageOpt("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", nDefaultDbBatchSize));
    }