Basic questions about string type and overflow memory

Question

What's the characters limit for a string variables? I was curious about it and I found out that it depends on the memory size available in the computer. Is that true? How can I catch that excepcion (exceding the string limit size)? My guessing is handled an overflow exception. But will this affect the performance in my computer?
I'm trying to export a java project to C# with Prolog that analize the amount of important words in a document or any kind of data. So the idea is to exclude the stop words and with the remaining words make a count of how many words are the importe data. So i put all the information in a string plus the command on prolog, but I was wondering how can I handled the overflow of memory.
In other words how many characters do I need to jam my pc? (It doesn't have to the exact amount)