You might need to make a few versions and ideally offer a vector for each of them if you're not the one who will implement these icons/buttons on your clients' sites.
It's easy to figure the size for the other social media buttons since you can create your own custom version at any size and by downloading their logos online on their own websites. What a lot of designers do is to offer a set of icons that goes from 16px, 32px, 48px, 64px, 128px, 256px in png , and as an extra you can add the vector version (.ai, .svg) since it's for your clients and more versatile too. The height of each icon is more a concern than the width but offering a square version is a good standard.
But most of the time the default dynamic versions of social media sharing buttons (eg. like, share, tweet widgets) are offered in different styles, some more squarish and others are more like flat rectangles for example.
For the "dynamic" version you'll probably need to look on the developer section of the brand guideline for each social media; in general they have a version that now fits well with the other most popular versions.
If you go on the "Layout Settings" of Facebook for example, you'll see they offer you the height for each type of button.
It varies between 20 and 80 pixels depending on what version our client wants to use.
When the size isn't written on their site, you can check the CSS of the button. For example, the small Twitter Tweet button seems to be 20 pixels height. Sometimes you can find this info in the code to embed the button too.
List for some popular social media sharing buttons/widgets and brand assets: