<dx:BootstrapGridView ID="GridViewServerMode" runat="server" DataSourceID="EntityServerModeDataSource" KeyFieldName="ID" AutoGenerateColumns="False"
    OnCustomColumnDisplayText="GridViewServerMode_CustomColumnDisplayText" OnSummaryDisplayText="GridViewServerMode_SummaryDisplayText">
    <Columns>
        <dx:BootstrapGridViewTextColumn FieldName="From" />
        <dx:BootstrapGridViewTextColumn FieldName="Subject" />
        <dx:BootstrapGridViewDateColumn FieldName="Sent" Width="220px" />
        <dx:BootstrapGridViewCheckColumn Caption="Has Attachments" FieldName="HasAttachment" Width="150px" />
        <dx:BootstrapGridViewTextColumn FieldName="Size">
            <Settings AllowAutoFilter="False" />
        </dx:BootstrapGridViewTextColumn>
    </Columns>
    <SettingsPager NumericButtonCount="4">
        <PageSizeItemSettings Visible="true" Items="10, 20, 50" />
    </SettingsPager>
    <TotalSummary>
        <dx:ASPxSummaryItem FieldName="Size" SummaryType="Sum" />
    </TotalSummary>
    <GroupSummary>
        <dx:ASPxSummaryItem SummaryType="Count" />
    </GroupSummary>
    <Settings ShowGroupPanel="True" ShowFooter="True" ShowFilterRow="true" ShowFilterRowMenu="true"/>
</dx:BootstrapGridView>
<dx:EntityServerModeDataSource ID="EntityServerModeDataSource" runat="server"
    ContextTypeName="DevExpress.Web.Demos.LargeDatabaseContext" TableName="Emails" />

protected void GridViewServerMode_CustomColumnDisplayText(object sender, ASPxGridViewColumnDisplayTextEventArgs e) {
    if(e.Column.FieldName == "Size")
        e.DisplayText = FormatByteSize(e.Value);
}
protected void GridViewServerMode_SummaryDisplayText(object sender, ASPxGridViewSummaryDisplayTextEventArgs e) {
    if(e.Item.FieldName == "Size")
        e.Text = string.Format("Sum = {0}", FormatByteSize(e.Value));
}
protected string FormatByteSize(object value) {
    string[] sizes = { "B", "KB", "MB", "GB" };
    double len = Convert.ToDouble(value);
    int order = 0;
    while(len >= 1024 && ++order < sizes.Length)
        len = len / 1024;
    return String.Format("{0:0.##} {1}", len, sizes[order]);
}