I used this answer, with a little bit help of Sublime 3 Help, I managed to port solution and change font size (and other attributes) of sidebar in my Sublime Text 3.
But how can I change (to larger) font size used by ST3 tab strip? If it is possible at all. I find filename tabs drawn with 10 pt font size quite not so comfortable on FullHD resolution screen. At least for my eyes.
As outlined in the answer you linked to, find your <theme>.sublime-theme file. Open it up and search for tab_label. The section should look something like this:
{
"class": "tab_label",
"font.size": 12,
"font.face": "Ubuntu"
},
Modify it according to your preferences and save the file. You should see the changes immediately.
To change the tab dimensions you can use the following section:
{
"class": "tabset_control",
"tab_height": 42,
"tab_width": 160,
"tab_min_width": 48
},
Note: Larger font sizes (e.g. > 12pt on my system) will get cut off, even if the tab height is adjusted accordingly. This is a well-known bug.