there are no real limits on the size of the string on the latest release.
to deal with lv2:Parameter types (often called lv2 patch parameters) we need to use lv2 atom ports and set the buffer type type to patch message.
then each parameter needs to be marked either writable or readable.
the “notes” plugin is a good example for how this works in terms of the UI behaviour, but its code is not the most readable.
There is lv2/plugins/eg-params.lv2 at master · lv2/lv2 · GitHub which is slightly simpler and shows how to use different types.
And also GitHub - x42/property_example.lv2: LV2 Property/Parameter Example Plugin which is even simpler but does not deal with strings.