Apex Coding on Mass Updating Records with a Custom List Controller .....

Class:
============
public class selectedSizeWorkaround {

    ApexPages.StandardSetController setCon;

    public selectedSizeWorkaround(ApexPages.StandardSetController controller) {
        setCon = controller;
    }

    public integer getMySelectedSize() {
        return setCon.getSelected().size();
    }
    public integer getMyRecordsSize() {
        return setCon.getRecords().size();
    }
}

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Page:
=====
<apex:page standardController="Opportunity"
    recordSetVar="opportunities"
    extensions="selectedSizeWorkaround"
    showHeader="false"
    id="muopp"
>
    <apex:form id="muform">
        <apex:pageMessage summary="Selected Collection Size: {!mySelectedSize}"
            severity="info"
            id="mupms"
        />
        <apex:pageMessage summary="Record Set Size: {!myRecordsSize}"
            severity="info"
            id="mupmr"
        />
        <apex:pageBlock title="Opportunity Mass-Update" mode="edit" id="mub1">
            <apex:pageMessages />
            <apex:pageBlockSection id="mus1">
                <apex:inputField value="{!opportunity.stagename}" id="stagename">
                    <apex:actionSupport event="onchange" rerender="muselectedlist"/>
                </apex:inputField>
            </apex:pageBlockSection>
            <apex:pageBlockButtons location="bottom" id="mubut">
                <apex:commandButton value="Save" action="{!save}" id="butsav"/>
                <apex:commandButton value="Cancel" action="{!cancel}" id="butcan"/>
            </apex:pageBlockButtons>
        </apex:pageBlock>
        <apex:pageBlock title="Selected Opportunities" id="muselectedlist">
            <apex:pageBlockTable value="{!selected}" var="opp" id="mutab">
                <apex:column value="{!opp.name}" id="oppname"/>
                <apex:column value="{!opp.stagename}" id="oppstage"/>
            </apex:pageBlockTable>
        </apex:pageBlock>
    </apex:form>
</apex:page>
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