If you want to display sizes in KB most of the time, but want a one-click way to toggle them into exact byte sizes, this button will do that:
(See: How to add example buttons to your toolbars and menus.)
<?xml version="1.0"?>
<button display="both">
<label>Toggle Size Format</label>
<icon1>#default:filetype2</icon1>
<function type="normal">
<instruction>Set COLUMNSTOGGLE=sizeauto(1)</instruction>
<instruction>Set COLUMNSTOGGLE=size(1)</instruction>
</function>
</button>
That's what I use. I like this way because the exact byte sizes are a pain to read most of the time, but sometimes they are needed (like you say) and when they are they're just a click away.
The button assumes that the sizes are shown in the second column, which is usually true. If that's wrong you just need to adjust the number in brackets.
On the other hand, if you want to see the exact byte sizes all the time, be sure to read the Folder Formats FAQ.