so I have made a CA account just to post this as it took me ages to fix this issue and I like to help.
I am not very computer literate but I love the total war games so when I got Atilla and I couldn't change the resolution I went on the good ol' google and I couldn't find anyone plainly telling us how to fix it, most of it was just in computerese and when it wasn't it only fixed half the problem.
this is how to fix it for dummies.
on STEAM, left click on the game, got to properties and in the launch options type -w 1600 -h 900.
this allowed me to actually play the game but it wasn't right and I couldn't change it to anything else, now what i did was check in windows what my pc's resolution is set to. I then went back onto the game and set it to that in OPTIONS >> GRAPHICS.
hopefully your smarter than me but it took me a good bit to "click-on" (pardon the pun) to click the tick to change it to said resolution
it then previews that resolution and i promptly clicked the tick to change it to the correct one
works great now. hopefully this help other idiots like me
All the best in the new year x