[reefland]

In /customer/main/product.tpl ( version 4.1.8 ):

Find:

{elseif $product.force_image_type eq 'W' && $product.variantid}

And

{if $active_modules.Magnifier ne "" && $config.Magnifier.magnifier_image_popup eq 'Y' && $zoomer_images ne '' && $js_enabled eq 'Y'}

Replace the code in between these two lines with:

{if $product.image_x > 380}
{include file="product_thumbnail.tpl" productid=$product.productid image_x=380 product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail" type="W"} 
{else}
{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.image_x image_y=$product.image_y product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail" type="W"}
{/if} 
{else}
{if $product.image_x > 380}
{include file="product_thumbnail.tpl" productid=$product.productid image_x=380 product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail" type="P"} 
{else}
{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.image_x image_y=$product.image_y product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail" type="P"}
{/if} 
{/if}

Replacing 380 with the max width you want.