This is fairly self explanatory. Below, select the character set and password bit (length) you want to use from the drop-down box. These contain some defaults. If you would like to specify a different value than those contained in the drop-down, simply select "specify" and specify your desired password bit or character set in the provided textbox.
Valid lengths for bit are from 1 to infinity. Although you could potentially generate as many random characters as you want, efficiency and processing speed will eventually slow down to the point it becomes virtually unusable.
Alternatively, you can select "specify" from the drop-down box and then fill in the textbox with a string you want to use in place of the default character sets.