Copy this to the clipboard, then click "Paste" under Preferences / File Display Columns / Evaluator Columns:
<?xml version="1.0"?>
<evalcolumn align="1" attrrefresh="no" autorefresh="yes" category="size" customgrouping="no" foldertype="all" header="Size" keyword="SizeMB" maxstars="5" namerefresh="no" reversesort="yes" title="Size (MB)" type="0">if (!IsSet("size")) return "";
if (operation == "sort") return size;
return size as %.02mb;</evalcolumn>
That will add a "Size (MB)" column in the same place the built-in "Size (KB)" one is.
Easier to read code (all contained in the XML above):
if (!IsSet("size")) return "";
if (operation == "sort") return size;
return size as %.02mb;
For other formatting options (e.g. more or fewer decimal places), see the As operator.