1.) Navigate to Project > Settings > Desired Capabilities > WebUI > Chrome:
2.) Click the Add button, and set Name = args, Type = List
3.) Under the Value column, click the ellipses.
4.) In the List Property Builder, click the Add button, then set Type = String, Value = window-size=x,y
5.) Set x,y in the value above to your desired dimensions, for example 1920,1080:
6.) Click OK, then OK again.
7.) Do steps 1-6 for firefox as well.