What fan size do I buy?

Solution
Front: 2 x 120 / 140mm fan
Top: 1 x 120 / 140mm fan (1 x 120mm FN V2 Fan Included)
Rear: 1 x 120mm fan (1 x 120mm FN V2 Fan Included)

So if you want all fan slots occupied, buy 2 120mm/140mm fan for the front. You can replace the stock fans in the back and rear to make your theme go together, which will make one more 120mm fan for rear and 120mm/140mm fan for top (or any other permutation and combination of that).
Front: 2x 140/2x120mm
Top: 1x 140/120mm (1 x 120mm FN V2 Fans Included)
Rear: 1x 120mm (1 x 120mm FN V2 Fan Included)

The case will support up to 4 fans, 2 are included

You can use either all 120mm or 140mm in the top and front. The rear is strictly 120mm
 
Front: 2 x 120 / 140mm fan
Top: 1 x 120 / 140mm fan (1 x 120mm FN V2 Fan Included)
Rear: 1 x 120mm fan (1 x 120mm FN V2 Fan Included)

So if you want all fan slots occupied, buy 2 120mm/140mm fan for the front. You can replace the stock fans in the back and rear to make your theme go together, which will make one more 120mm fan for rear and 120mm/140mm fan for top (or any other permutation and combination of that).
 
Solution
mbilal2, I have a really weird question but if you look in the rear of the case and look below the stock fan, would a fan go there too? I also wanna replace all the stock fans so it would go with my build color scheme, thanks!
 


No you can't fit a fan there (unless you use zip ties which I don't recommend). Plus the graphics card will come in the way.
 


To occupy all slots and replace the stock fans, yes. Again the case has 4 fan slots (according to manufacturer's website) with two stock fans included. You don't need to have all 4 fans, just one at the back is the bare minimum. But if you want to light your case up, need better cooling and have the cash to spare, buy 4 fans, take out the two stock fans and put in your own fans.

3 of them can be either 120mm or 140mm but the fourth one has to be 120mm.