What the best resolution of the ViewPort Size for this kind of game ?
Im using 16:9 / 854 by 480
Can I use 16:9 / 426 by 240?
You can use whatever you want! It's as much an artistic choice as anything else. Smaller resolutions will obviously perform better, but you may not like how things look at too low-res. It's a balancing act. Sometimes when I start a pixel art game project, I'll make a few test images at different resolutions to see what looks the best and what I'm the most happy with.
if I increase to 1920 by 1080 , it will force me to have images larger than I should use in a small resolution correct ?
I mean I want to have a game with a good performance , with nice resolution and that fit all devices .
A noble goal, but you might find it's a good idea to get more specific about the devices you want to target. When you say "all devices", are you including mobile apps? You will likely have difficulty finding a resolution that performs well on mobile while also looking good on desktop PCs. And more to the point, few types of games are equally suited to PC, consoles, and mobile (most kinds of games work best on one platform or another). So rather than trying to please literally everybody, you may find it helps to pick one target platform and work towards that.