What is a safe Maximum Stack Size or How to measure use of stack?

I have an app with a number of worker threads, one for each core. On a modern 8 core machine, I have 8 of these threads. My app loads a lot of plugins, which also have their own worker threads. Because the app uses huge blocks of memory (photos, eg. 200 MB) I have memory fragmentation problem. The problem is that every thread allocates the {$MAXSTACKSIZE ...} of address space. It's not using the physical memory but the adress space. I reduced the MAXSTACKSIZE from 1MB to 128KB and it seems to work, but I don't now if I'm near to the limit. Is there any possibility to measure how much stack is really used?