In my post.asp it's in line 915. Have a look for
<input class=newlogin maxLength=""50"" name=""Subject""
and change maxlength to whatever.
In my post_info.asp it's in line 878. Have a look for
if Len(Request.Form("Subject")) > 50 then
and change it to the allowed value.
Note that the db field for subject is limited to 100 chars (at least in access)