Hello,
I will show you step by step how to custom your CSS, That will solve your trouble.
Take care to follow precisely ALL steps.
1° Go to the Components -> HikaShop -> Configuration
2° Click on "Display" tab
3° Click on CSS
4° Here, on Front-end CSS File you must have selected "custom" BUT if not, select it!
5° Click on the little pen/ "edit" icon
6° Copy & Paste this code :
/* ADD CSS CODE HERE !!! */
@media only screen and (max-width:400px) {
select #hikashop_product_option_0 {
width: 100%;
}
}
/* CUSTOM END */
AND don't forget to save !!!
If I'm right your issue will be solve with this.
Regards,