If you simply want the current stack size, you could declare a variable at the top of main(), take its address, and compare it to the address of a variable declared at wherever you define "current" to be. The difference should be the approximate size that the stack has grown.
If you want to know how much memory is reserved for the stack, you can check /proc/[pid]/maps, which has a region marked as [stack]. For example, my atd process has:
7fff72a41000-7fff72a56000 rw-p 00000000 00:00 0 [stack]
0175b000-0177c000 rw-p 00000000 00:00 0 [heap]
which gives you an idea.
A neat trick that a friend shared with me when I wanted to know the maximum size of stack that my program used was as follows. I'll present it here in case someone finds it useful :)
1) In a function called near the beginning of main(), use alloca() or a very long array to scribble 0xDEADBEEF or some other such unlikely constant over as much of the stack as you expect could be used. This memory will be "freed" when the small function returns.
2) At the end of main, again use alloca() to grab a region of memory and "search" down through it for whatever magic constant you used to scribble (you might try to find the first block of 64 of them or something to skip over regions of memory that may have been allocated but simply never used), and where that pointer lands indicates your maximum stack usage.
Not perfect, but it was useful for what I was doing!
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