Sizable means a significant amount of something. Outside of North America it is spelled sizeable. The spelling change extends to the adverb form sizably and sizeably. Be careful to use the correct form (i.e., do not use the adverb form when the adjective form is required.)
Interestingly, the word peaked in popularity in the 1970s and since has dropped off. This is true of both spellings.
Examples
There’s a sizable pool of mainstream Republicans here waiting to be courted, and in a crowded presidential field, a candidate can win the state with a plurality — and without unified support from social conservatives. [CNN]
Coming together in 2009, they share a passion to create full-featured homes in the 600 too 1,000 square foot size. Easy to heat, easy to maintain and easy to afford are what they think a sizable number of Canadians want but can’t find. [The Chronicle Herald]
In fact, the opposite is true; a sizeable portion of the profits of government businesses that provide such services as rail, ports and electricity flow back to Treasury coffers as dividends, and are used to fund the provision of additional social infrastructure and services. [Sydney Morning Herald]
Kapoor has ordered Tom Yum Goong soup, in which sizeably large prawns are dunked, and wok fried fish with crispy ginger (Pla Phad Khing) for starters and we start the meal with gusto. [Business Standard]
More of that on the following pages but what both press conferences clearly illustrated that the difference between both teams is sizeably more than the five point margin with which Europe romped to glory in the glen. [Daily Record]