Jon's layout suggestion (reply above mine) is the simplest way, but if you find that changes more things than you want it to, you could instead replace the Tools > Find Files menu item with this:
(It doesn't have a hotkey defined, so if you do replace the old menu item, give the replacement the Ctrl+F hotkey by editing the menu item after adding it.)
For reference, it runs this script, and you can edit the 3rd line to change the minimum width:
function OnClick(clickData)
{
var minWidth = DOpus.DPI.Scale(800);
var cmd = clickData.func.command;
var lister = cmd.sourcetab.lister;
var width = lister.right - lister.left;
if (width < minWidth && !cmd.IsSet("UTILITY=Find,Toggle"))
{
cmd.RunCommand("Set LISTERSIZE=" + minWidth + ",+0");
}
cmd.RunCommand("Set UTILITY=Find,Toggle");
}