Currently Online 
It really depends on your budget. If you are invested in 4k gaming then it will require a lot of grunt. Especially in the GPU. If you can afford a RTX2080 card then sure, it might be worth it. Also, with 9th gen consoles you will get full use of the extra fidelity. And if you have an Nvidia GPU there's DLSS which drastically improves performance by up scaling at lower resolutions - see DLSS 2.0 patch for Control on YT.
The sweet spot imo is 1440p at the moment when it comes to value and with raytracing becoming the norm, you can push for even higher frames while keeping up fidelity for longer. And not lose much clarity since you are sitting closer the PPI difference won't be as apparent.
Size is dependent on how far you sit from your desk and what is comfortable. For me anything over 32" 16:9 is way too much as the top and bottom parts is out of my peripheral vision and requires me to look up or down and or move my neck. Whereas my current 21:9, 34" ultrawide is about as tall as a 27" and as long as a 40" screen which is almost perfect for sitting in front of my desk.
High refresh rates I wouldn't say is that important unless you play competitively - say games like Rocket League, CS Go etc.
