slightly worried about adding too much code in Write, txBuf, … as this could have a performance impact (but I haven’t measured it, it’s likely to not be impactful).
Haven’t given much thought to this part of the PR, so I don’t know if we can handle this differently.

We could change the signature of Write method to Write(buf []byte, size uint32). This would mean we would only need to check in hyperkitProtocol.Write for uint16

I suspect these lines

yes, that's right. We can perform conversion here and then call the Write function. We can return an error in case of an invalid value. However, there's still hyperkitProtocol which needs uint16. We could also skip Write in case of an invalid size (but I think this is not correct):

        if size > math.MaxUint32 {
		log.Errorf("size exceeds max limit.")
	}  else {
	        binary.BigEndian.PutUint32(buf, uint32(size))
        }

This nosec annotation is not needed on my system (I pushed this change to your branch)

Interestingly, tap.Switch no longer seem to be using the mtu, all use is now in the LinkEndpoint. This could be cleaned up in a followup PR.