Shoutbox

You can't increase the max file size without patching Messenger.

However, be advised that the limit is there for a very good reason: speed. The larger the files get the slower it becomes to save each line of text. Even with the current max size you sometimes even notice this delay.

You could combine existing logs, but that requires some knowledge of XML because it isn't as simple as just copying one log after the other into a new file. There is currently no tool that I know of which can automatically merge native Messenger xml logs.