There seems to be a max length to how many chars a string can hold. In my case it’s being populated with HTML via JSON and at around 1500 chars it crashes.
The maximums (both practical and theoretical) are far higher. Theoretically, it's 2^31 - 1 characters. In practice, you'll be limited to available memory, but even on a mobile system you'll have more than 1.5KB.
You have a different issue. Use JSON Lint to first check that you have valid JSON.