random width lumber

Hardwood lumber comes in random widths. For visual, here's a typical pile of red oak. Each board is a different width.

Most widths are 5" to 7" in most woods in most thicknesses. (There are always exceptions!)

No boards are any narrower than 4", except in unusual circumstances. 

The amount of wider choices declines once we start talking about 8" and wider. Rarely are any boards wider than 10". 

Basically the vast majority of 4/4 lumber is 5" to 7" wide. Thicker lumber, such as 8/4, is often mostly 7"-10".

lumber widths available for hardwood, a bell curve graph

Here's a visualization of what you can expect in lumber widths. Most are 5" to 7". Very little is 12" or wider, and nothing is narrower than 4".