...Like if I did it on say 1080x1980 then output it into 300 x 100, will that affect anything?...
Well, make life easy for yourself and keep the ratio the same....so 600 x 200, or 900 x 300, or 3000 x 1000....whatever......but not just any old size.....think ahead!
When you come to save it out save the job as a PSD file first, then use 'Save For Web' and change the image dimensions FIRST, (to 300 x 100), before anything else.
Its a good move to use the '2-up' feature to compare the original and the 'optimised' versions of your image.....there can be a big difference between formats.
Then choose your format.....png, jpg, etc.......changing any settings you may need.....none I shouldn't think really, then save it.
You can always open the PSD file again if you want to make any changes.....then save out again to 300 x 100.....or whatever.
Regards.
MrTom.