Product Page image Size

I would like the image size on the product page to be bigger, I saw in another post how to have the full size image showing instead of the thumbnail, but what I need is a image that is bigger than the thumbnail but not as big as full size. And I cant change the thumbnail size because the rest of the thumbnails in other sections would be too big.
Is there a way to set the size of the image in product page to be bigger than the thumbnail?

?¿ but in my question I said "I cant change the thumbnail size because the rest of the thumbnails in other sections would be too big" and you say "The images size on the product page can be changed in the Files tab of the configuration." but isn´t this changing the thumbnail size? because when I change the image size in the File tab in configuration nothing happens? it seems as if the product page is only displaying a thumbnail because this is the html code:

<div id="hikashop_product_image_main" >

<div id="hikashop_main_image_div" class="hikashop_main_image_div">
<div class="hikashop_product_main_image_thumb" id="hikashop_main_image_thumb_div" >
<a title="gonz" rel="{handler: 'image'}" target="_blank" href="/joomla/media/com_hikashop/upload/gon-001.jpg" onclick="SqueezeBox.fromElement(this,{parse: 'rel'});return false;" id="hikashop_main_image_link">
<img src="/joomla/media/com_hikashop/upload/thumbnail_100x81/gonz-001.jpg" alt="gonz" id="hikashop_main_image" style="margin-top:10px;margin-bottom:10px;display:inline-block;vertical-align:middle" height="100" width="81" />
</a>
</div>
</div>
<div id="hikashop_small_image_div" class="hikashop_small_image_div"> </div>
</div>

As you can see there is nothing in the hikashop_small_image_div ?? is this a bug?
Please help.

Ok, so you are telling me that there is no way of changing the size of the image displayed in the product page without changing the thumbnail size?? that not good because if I change thumbnail size the product page looks good, but the product list page for categories looks bad because thumbnails are too big there.
How can I have for example 400px image width for single product page and 150px image width for thumbnails in product_list page?
there must be a way? do you understand what I am asking for? sorry if I am not explaining myself correctly.

that is basically correct, its one a only a very few issues that have to be worked around, it would so make much more sense to control these seperately.

YES, Thank you! that was it. Thank you so much for your help.

However, now I have a new problem.
the image in my product page is now big and nice, but the product images shown in cart in the chackout page is now too big. Its as big as the product page and I need them to be small thumbnails, since its just a cart showing the products.
How can I do this??