No such thing a too big! Many varieties have leaves 3-4+' high (not counting the scape) and thick necks are often indicative of large bulbs.

Harvest when about 60% of the leaves are yellow/brown - you want to have at least three green leaves as each leaf represents a wrapper layer on the bulb.

TomNJ

Correct, there is no such thing as too big..Music, german White, Russian Red are all very big plants. Do not be afraid of a big plant...it's a good thing. I like to pull them when there are 4 green leaves left so I can pull one off and have a nice clean bulb.

If that doesn't matter then as Tom said dig them when you have three green leaves left and hang them in a shady spot for say three weeks to cure