You want to protect the drywall in any "wet" bath. A lot depends on
the look and feel of the bath, but matching the splash to the counter is
the simplest . It does look far more stylish if you go higher than the
standard 3 inch plus. Just consider any wall outlets, before your
template. A six inch back and side splash looks great, leaves plenty of
space for mirrors, trimmed or not. Outlets can even be cut in to
"disappear". As below

Thank you Jan. Like the idea of back splash same material as counter top and wider than standard 3+"

If you go that route, use a thinner splash than the 3cm countertop. I dislike the side view of a chunky 3cm backsplash and you certainly wouldn't want 6" of it. If it's not available in a thinner material, just do the back wall in the counter material. If your side splash needs protection because the sink is close, use tile for the entire backsplash.

Had not considered what countertop material would look like from the side. Thank you!

Some jurisdictions require a backsplash, so you can’t just do wall. You should find out from your contractor what those requirements are and then design for that. Even if your inspector does not require a backsplash, think about whether you really want to deal with water splashing directly on a normal, non-waterproofed wall. I personally do not like the look of using a few inches of the counter material behind the sink, but you can do that. I prefer tile wainscoting all the way around. If you do not want to do that and really like the look of just regular wall, another option is to get a sink (like a console or pedestal) that has a backsplash as part of the sink. But again, whether that has enough inches to meet code will depend on your jurisdiction.

Thank you. I will check with our contractor.

It looks FINE from the side.............: ) sorry, yes it does. Why on earth have the same material meet a chunkier version of it? The side is as polished as the front edge of the counter. More overthink isn't necessary

Will check out samples and see what looks best. Thanks!

We used the same material as the counter, marble. We did a 6" back splash behind sinks with mirror starting immediately above splash, and 9" around tub deck with paint above.