I have lot of empty space in the Find window.
Type, Date, Time and Size could be listed side by side.
Putting Regular expressions etc next to the Wildcards part would save some space to. My guess is doing it that way saves 66% height.
Maybe it's not so for al screens, but with more and more large screens, and (ultra) wide screens it's something to consider?
Maybe an adaptive layout?
Not when they're in use, which reveals extra controls next to the dropdowns:
The right half could be made more compact but it's the left half that determines the minimum height.
A dynamic layout could work, but then you'd have to keep scrolling or resizing the panel when turning options on and off, which might be a pain. (Advanced mode is already like that, of course.)
The labels are much wider in some languages, which the layout has to cope with.
Comparing 12 and 13, the layout in 13 is actually smaller than the one in 12 if you set it to use the 8pt font that 12 used, despite there being more in the new panel.
13 defaults to a 9pt font, which makes it about 1 line of text taller than before (but packing in a lot more things than before as well). That font size change is the biggest factor between the two versions. (At least at 200% DPI scaling. I haven't checked other scaling where fixed-size borders may be more of a factor.)
I would also like to see an improvement.
In all 3 functions (search/sync/dub) the confirmation button is anchored on the right for large monitors
it would be better if it were next to the movable separator.
Not when they're in use, which reveals extra controls next to the dropdowns:
That's correct but I hardly ever use them. So an option would be that they expand, as shown in your picture, when they are acutually used.
On my PC I use 43" screens and even then I'm obsessed with efficient space usage.
Get's even worse when on my laptop. 
The right part really needs to be made more compact, laterally speaking. The old way worked just fine, if I remember correctly, and I didn't have to widen the lister window to get to the "find" button.
You can resize the splitter between the left and right halves.
Yes, but when I drag the splitter as far as it can go to the left, I still need to keep a very wide lister just to see the find button. The "Name" and "Text" field on the left don't need to be that wide, most of the time. There's be some room to gain here. My lister, with the Find button visible, looks like this.
