How to change the size of multiple text boxes in Side by Side questions?

I am asking respondents to insert the month and year they were born in a side by side question as I prefer this layout. However, I want to choose the size of the text boxes beyond 'small, medium, large', I have found some javascript to change the size of both boxes:
jQuery("#"+this.questionId+" input[type='text']").css("width","3.5em")
});
Which looks like the below:
However, I want to know how to amend the the script so that they are different sizes.
Thanks!