I've seen a few people ask about the size of a fully compiled source 
tree.  Here is the listing of my system.

gabriella% df
Filesystem  1K-blocks    Used   Avail Capacity  Mounted on
/dev/wd0a        9775    7185    1612    82%    /
kernfs              1       1       0   100%    /kern
/dev/wd0e        4811     567    3762    13%    /var
/dev/wd0f       19193       3   17270     0%    /tmp
/dev/wd0g      251634  180267   46203    80%    /usr

I have a full -current src tree with object files in /usr/src
There are virtually no user files, but Xfree 2.1 is installed.
This should give you a good estimate of the space required.

I did remove all ports other than i386 in /usr/src/sys/arch


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