Your limit is a function of the resources you want your node to consume. Above that limit, your node either rejects large blocks that are spam (not accepted by the longest chain), or drops off the network because it can't keep up.
So does BU follow the longest chain or not?
If not, than it obviously can't work, as it was pointed out a million times already - everybody will fork off randomly according to their limit.
If it follows the longest chain, then your limit doesn't matter, as eventually you will have to switch (otherwise, again, you will be forked off).
Since BU can only work if it follows miners, your personal limit becomes irrelevant: if you insist on setting it, you will just hurt yourself and will still have to eventually switch if you plan to continue using Bitcoin.
This limit cannot be used as a signal to miners because of Sybil attacks, so it's pointless.
Yet BU proponents push it as the main feature of BU
