If I were you I'd go onto one of the allotment sites or speciality "giant vegetable" sites for more info.

Basic onion needs, to start:
Full sun.
A soft, rich, sandy soil.
Plenty of water, but good drainage.
Fertilizer rich in nitrogen and potash, a 3-1-2 proportion is ideal--such as 15-5-10, or the like.
An early start--the longer the plant grows leaves before day length starts the bulb growing, the bigger the bulb will be.
Beyond that, the chances of having giant onions will depend on the variety you use, how well it fits your individual climate, the weather, pests and diseases, etc., which is where Chammomile's suggestion comes into play.