In terms of display size, a 15-inch screen is the sweet spot for a gaming laptop. You can buy larger 17-inch displays, but this can jack up the weight to way beyond 5 pounds. We've seen 12-pound "portables" in the gaming sector that will definitely weigh down your backpack. We recommend at least a full HD (1,920-by-1,080-resolution) screen. Larger displays are capable of giving you higher-than-1080p resolutions, but choose wisely, as a resolution of QHD+ (3,200 by 1,800 pixels, and uncommon) or 4K (3,840 by 2,160 pixels, a bit more common) will boost the final cost twice: first for the panel, and second for the higher-quality graphics chip you'll need to drive it to its full potential.
You'll see guides from 2017 or even from this year still recommending laptops with Nvidia's 900 series mobile GPUs, for example the 950M or 960M. Don't settle for these. Nvidia's Pascal architecture in its 10-Series graphics cards is a huge leap forward in performance and power efficiency. You may save $50 to $75 on these systems, but you won't save yourself a few years of disappointment.
Given that high-end components tend to drain battery life, don't plan on taking any of these gaming rigs too far from a wall socket very often. Cutting-edge ports like USB-C and Thunderbolt 3 are beneficial now, and will only be more so down the road, but look for at least two USB 3.0 ports so you can plug in an external mouse and a hard drive for your saved media files. If you mean to attach a VR headset to your GeForce GTX 1060-or-better rig, look for the right loadout of ports to accommodate it; you'll need a well-placed video out and enough USB ports for the hydra-head of cabling. Other video ports, like HDMI or Mini DisplayPort, will be helpful if you want to play games on an external display, but aren't absolutely necessary if your laptop's screen is large enough.