answering the question: no sorry you cannot increase/decrease the line width using a shortcut [or cursor keys] in the way you can in lots of other software [eg inkscape]

as Acorn kindly pointed out you can click and scroll, but you are limited to the values in the drop down which are frequently not helpful - you can preset these, but you need to leave the program and use XaReg utility [or edit registry] every time you change them, and you are still limited to the number of steps in the dropdown

to call this clunky is an understatement