Hello Ted, 

It is not very clear what your asking is for, so I will try to cover the most of the possible scenarios.

I have used the following online samples as an example:
http://www.igniteui.com/grid/checkbox-column
http://www.igniteui.com/grid/row-selectors  

You could easily inspect the css classes applied to any rendered HTML element using the available browser’s Dev Tools.
What is more, using this approach it is possible to change the CSS and observe changes in real time, as well as a detailed information where this CSS styles are located (in which file as well as the exact line in the file). 

I. For example, the responsible css for the checkbox size is currently located under Infragistics.css:71 

.ui-igcheckbox-small {
display: inline-block;
width: 13px;
height: 13px;
overflow: hidden;
}

Changing the width and height here will change the CheckBox size directly. 

However, the checked CheckBox image is depending on several styles, and its position and size should be changed as well, if the default Infragistics theme is modified. This change is required, in order to fix the checkmark position when the CheckBox size is changed. For example, the CSS responsible for the image position is located under Infragistics.theme.css:215 

.ui-icon-check {
background-position: -64px -144px;
} 

Also please notice there is other CSS involved, as the one specifying the checkmark image location àInfragistics.theme.css:80: 

.ui-state-default .ui-icon, .ui-state-hover .ui-state-default .ui-icon {
background-image: url( images/ui-icons_888888_256x240.png );
}

II. If you are using row selectors,
The CSS responsible for the checkBoxes size is located under Infragistics.css file. In particular -->Infragistics.css:68 line 

.ui-igcheckbox-normal {
display: inline-block;
_display: inline;
overflow: hidden;
width: 16px;
height: 16px;
}  

If it is required to change the checkBoxes column width, and not the checkBoxes size, I suggest take a look at the following option available. http://help.infragistics.com/jQuery/2014.1/ui.iggridrowselectors#options:rowSelectorColumnWidth  

This is the general approach. You could further look into the css files and tweak the necessary css styles, in order to achieve the desired styling.
Please let me know how these hints work for you!