From Eggdrop Wiki
To change the number of characters eggdrop allows for a handle, you must first ensure you have the source files you compiled from (This is the source files, the files that came out of the .tar.gz file you probably downloaded, not the compiled files located in ~/eggdrop). If you dont have the original source, thats ok, download the latest from www.geteggdrop.com and use those (this will also upgrade you to the most recent version of eggdrop if you do not already have it).
Eggdrop 1.6.20 and higher
You can just supply the desired handle length to ./configure like this:
Eggdrop 1.6.19 or lower
Now that you're in the ~/eggdrop1.6.X directory, move into the "src" dir, and edit the file "eggdrop.h" . Search for the term "HANDLEN" and change the value after it. In the 1.6.19 version, the default line looks like this:
#define HANDLEN 9 /* valid values 9->NICKMAX */
For instance, if you wanted to have a handle length of 12, the line should be edited to look like this:
#define HANDLEN 12 /* valid values 9->NICKMAX */
Do NOT remove the #, that is not a comment symbol like it is in Tcl!
After saving these changes, kill the bot, recompile the source, and restart the bot
(recompiling means running ./configure && make config && make && make install from the eggdrop1.6.X directory, just like you did when you compiled the bot the first time)