There are various schemes to address this. A simple one is to make the legtth prefix variable, nad use the high order bit use the highorder bit of each byte to indicate there is another length byte following the current one. This gives you 128 possible values for each length byte. The max string size for a single byte prefix would be 127 (2^(8-1), the max size for two bytes would be 16384 (2^16-2), etc.

It's not necessarily a good solution, there are likely ones that are much more performant, but many solutions exist, and a few have even been implemented in widely used languages.