You won't be able to get the `size` directly in `visualforce` email template, for that you will have to use the repeat tag to get the count and then you can build further logic based on that variable.

    <apex:variable var="sizeOfOpportunityLineItem" value="{!0}"/>
    <apex:repeat value="{!relatedTo.OpportunityLineItems}">
    	<apex:variable var="sizeOfOpportunityLineItem" value="{!sizeOfOpportunityLineItem + 1}"/>
    </apex:repeat>
    Number of Opportunity Line Items : {!sizeOfOpportunityLineItem}