Hi,
The properties are responsive ones, so in %. If you want to set a width in px, so you will loose the responsive.
To have a fixed width
1) For the height, you will have to increase the height of the div with the class "hikashop_product_name", and "hikashop_product_image":
.hikashop_products ul hikashop_subcontainer{ width: 248px !important; }
.hikashop_products ul hikashop_subcontainer .hikashop_product_name{min-height:110px !important;}
.hikashop_products ul hikashop_subcontainer .hikashop_product_image{min-height:120px !important;}
2)
.hikashop_products ul hikashop_subcontainer form{margin-bottom:10px !important;}
3)
.hikashop_products ul hikashop_subcontainer .hikashop_product_price_full{margin-bottom:10px !important;}