Documents and Their Sizes

Documents.apxc

public class Documents {
    public Document[] getDocuments() {
        return [SELECT Name, BodyLength FROM Document];
    }
}

Documents.vfp

<apex:page controller="Documents">
    <apex:dataTable value="{!documents}" var="doc">
        <apex:column headerValue="File Name" value="{!doc.Name}" />
        <apex:column headerValue="File Size (KB)" value="{!floor(doc.BodyLength / 1024)} KB" />
    </apex:dataTable>
</apex:page>