The average web-page fixed width is around 950px-1200px so you can take your pick. However if you plan on doing it responsive, I'd go up higher and do multiple mockups (one for deskop, one for tablet and one for smartphone).
Height is up to you. Something like 3000px covers it, your page shouldn't really be much longer in height any way, but you can always adjust that along the way if need be.