I'm building a computer for the first time, its main function will be gaming, and for now I'll just use one monitor. Will I need a 4gb or a 6gb graphics card?
Depends on what games your playing and what settings. If your putting a ton of third party mods and textures packs on, you'll want to opt for a 6GB video card, since it's going to eat through VRAM. If you arent going to be doing that, then a 4GB card is perfectly adequate. I Know quite a few people who are Using 1050 ti's and 290X's and they havent had any issues with a 4GB frame buffer. My one friend has been using his 980 for a year or two now and he hasnt run into VRAM limitations either.
If you're gaming at 1080p, 4GB will be just fine.
I would suggest that you link you build here so the guys can advise you on the complete solution.
I've been using 4gb and its working fine. But if you can afford a 1060 6gb that would be a good option imo. Especially since you say its for gaming.