Barabb4s

You Need edit this too:  src/char/int_guild.c  around line ~ 866
 
g->max_member = BASE_GUILD_SIZE + inter_guild->checkskill(g, GD_EXTENSION) * 6;  

@WhiteEagle just have the solution to your problem. If you have another error, please elaborate with screenshots if needed.
 
If you need to change Guild size no matter skill level, just change
#define BASE_GUILD_SIZE
to some high number