Column width

I am in my first hour of using the Site Origin Page Builder, and wish to thank you for this awesome product. Being new to this, perhaps I’ve overlooked something that should be obvious, but I cannot see how to control the column width by pixels instead of percentage. I made a 3 column section and wish to specify that the first column be 160 pixels wide, the third column 300 pixels wide, and the middle column to use whatever additional space happens to be available. How do I do this?