Hi Dan,
Sorry for the late response.
Note that you can already increase the width of the folder tree by dragging the separator but I guess you need to change the default width.
Open FileVista\FileVistaControl\filevista.asx and find these lines:
<div id="divLeftPane" style="position: absolute; top:0px; left:0px; width: 214px; visibility: hidden;" class="nonSelectableText"></div>
<div id="divPaneSeparator" style="position: absolute; top:0px; left: 214px; width: 6px; visibility: hidden;" class="separator nonSelectableText"></div>
<div id="divRightPane" style="position: absolute; top:0px; left: 222px; visibility: hidden;" class="nonSelectableText">
You can see that the width is set in style properties. You can increase the width of folder tree pane (divLeftPane) by increasing the value width: 214px , note that you should also increase the left positions of separator (divPaneSeparator, left: 214px) and right pane (divRightPane left: 222px) by the same amount.