If you can adjust Message Box correctly in Windows 8 (assuming the link I gave you gives you the ability to do that), it should adjust the size of the tabs, and the font size in them. It does the same for the personal bookmarks bar.
I made the adjustments in Windows 7 to Message Box, and increased the size from 9 to 19, bolded it, and italicized it, and the tabs are huge, the print is large and bold. Now a few things. Make sure you save (in all the places necessary) the changes you want, and also, after you save, where you have to save, close the browser and then open it again. I just did that and there's a huge change to in the size of the tabs, and the print in them. Also, a huge change in the size of the titles of the folders on the Personal Bookmarks Bar. And a big change in what's in the address bar. For people who can't read what's in the address bar, this definitely increases the font there. Now I have to restore the setting to something more reasonable.
Also, I increased the size of my Menus, and they help when I click on the Opera logo, to view my menus. Now they're a good size. None of this affected the size of the icons. I'm not sure what affects that.
Now be advised, these changes will affect all your browsers. But it's should be easy to reset this if you're unhappy.