The general CSS class will be
button svg.tc-image-button { width:30pt; height:30pt; }
Of course this will increase the size of all the buttons. So it ought to specified with necessory selectors.
Say you are transcluding the button to a tiddler tagged 'mytag', the CSS will be
.tc-tagged-mytag button svg.tc-image-button { width:30pt; height:30pt; }
Say you are transcluding it within a span class with html <span class="myclass">{{$:/core/ui/Buttons/save}}</span> the CSS will be
.myclass button svg.tc-image-button { width:30pt; height:30pt; }
All the core ui buttons have separate classes assigned to them. For eg: you want to increase the size of just the home button under a span class "myclass" the CSS will be
.myclass button svg.tc-image-home-button { width:30pt; height:30pt; }