[PhilJ]

David, you should be able to adjust the thumbnail sizes in general settings > appearance, then regenerate the thumbnail cache.

If you want to hard-code thumbnail sizes, while not advisable, you can do it like this...

skin/ability/customer/main/products_switchers.tpl (assuming you're using the grid/row switchers)

{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.tmbn_x image_y=$product.tmbn_y product=$product.product tmbn_url=$product.tmbn_url}

To...

{include file="product_thumbnail.tpl" productid=$product.productid image_x=150 image_y=150 product=$product.product tmbn_url=$product.tmbn_url}