Hi zoeatdemetra,
For the first question, you can edit the sizes in HikaShop > System > Configuration > Files.
And unded the Images tab, you can set the thumbnail width and product page image width.
To magnify when the mouse moves over the picture, it's not included in hikashop, but you can do it in JavaScript with an:
<img id="hikashop_main_image" [...] onMouseOver="magnify();"/>
In your magnify function, create a display with the same src as your picture.
Maybe there is already existing plugins in Joomla for that.