From a thread on the topic;
http://www.tomshardware.com/forum/id-3384170/gtx-1070-monitor-recommendation.html
1440p is ideal on a 27" monitor with a general .233 pixel pitch. A 24" is typically around .27. These mean the lower the decimal value, the more pixels per inch/density/clarity you will get. So, a 1920x1080 monitor at 24" size will be more crisp than a 27" monitor at the same 1920x1080.
Buying a monitor is a potentially soul-rending experience of patience and research. 1. Budget. 2. GPU...yours is a nice option for 1440p and as noted, solid for 144hz. 3. Use: Ratio of Games-to-Productivity 4. Performance. Guys will argue this order of importance but you have to consider them all.
With a 1070, I would go for a nice 27" 1440p monitor. If you are an FPS heavy player, go for a monitor over 75 hz refresh, toward the 144 range. If you do mixed use, a 75hz might be fine for you. Then there are the panel types... TN vs IPS... which is essentially "speed" or "color/blacks". There are so many factors and elements it makes a mind wobble.
Monitornerd.com has some good guides and opinions and you'll find they are a dime a dozen when you ask around. I would say set your budget, find one that fits your use, and get the best deal you can. I personally think chasing 4k is a lost cause at this point and your 1070 will only make you cry. As well, 1080 is great, fast and more than enough for the average user but it is going to become outdated and replaced by 1440 in the very near term. Ultra-wide monitors are getting better and cheaper and will seduce some users, but I don't think they're best for all-around use at this point...games+productivity, but to each his own.
For $500, watch for sales/check pcpartpicker and you can get a nice selection of quality 27" 2k/1440 monitors from 60 to 144hz. It almost becomes a beauty contest ;p I settled on an Asus MX27aq a month ago as it suited my mixed-use needs and looked good doing it. It is a fantastic IPS screen without a blemish and my GTX770 drives it more than well enough for my uses at this time...until I go in on a 1080 or AMD Vega equivalent this summer.