Squid needs to be configured before the first usage. That's not something webmin can do. In general, it is faster, safer, more flexible to use the shell for server maintenance. So I recommend pulling off the band-aid quickly rather than slowly and opening up the terminal. Webmin is good for assistants once the head sysadmin has set everthing up, but its usefulness ends there. Once you're running you should be able to go back to webmin.
Only a few things need to be set before use, though the other options are nearly endless. Look in the manual page and the online documentation for squid for information about the directives cache_dir, acl and http_access
The size is given in MB. So the 100 in the default is 100MB. Maybe that is too small.
# Uncomment and adjust the following to add a disk cache directory.
# cache_dir ufs /var/spool/squid3 100 16 256
Then there is the question about who to allow access to. There acl and http_access come into play. Look through the acls to see what the pre-set options are. Then set http_access
Once it is configured, you need to create the cache. That is done with the -z option.
# INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
# uncomment this line to allow access to the localnet as defined by the acl above it
#http_access allow localnet
That's a lot but now it should just run on its own and maybe it is time for webmin.
sudo service squid3 stop
sudo squid3 -z
sudo service squid3 start
But you can always watch the access logs real time with tail.
tail -f /var/log/squid3/access.log