In order to assign adjust width for columns in apex:panelGrid, we have to create different CSS styles for columns and we have to assign it to “columnClasses” attribute.

Sample Code:

<apex:page >

<style type=”text/css”>
    .col1 {width:100px; text-align:center;}
    .col2 {width:150px; text-align:center;}
    .col3 {width:200px; text-align:center;}       
</style>

<apex:pagemessages />
<apex:form >
    <apex:panelGrid columns=”3″ columnClasses=”col1, col2, col3″ border=”1″ cellpadding=”5″ cellspacing=”5″>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
        <apex:outputText >Sample</apex:outputText>
    </apex:panelGrid>
</apex:form>
</apex:page>

Output: