For font size, it is absolutely possible. Here are a pair of screenshots showing that I have Default Style at 10pt and Line Number Margin at 5pt:
3005c100-1f3a-43c3-a01f-9a7d6e2e9e0f-image.png
5ff071c8-e49d-4045-8640-5ab9e07d2bad-image.png
Or here’s one with Brace Highlight Style set to 5pt – the first screenshot shows the braces not highlighted, at 10 pt; the second shows them highlighted, at 5pt.
f5903d88-cbfe-4499-9fc1-8861c82d7779-image.png
a2628fe2-f039-42e8-bb35-e4e214adb8f3-image.png
However, you said,
line spacing
Sorry, line spacing is not individually configurable. Notepad++ computes the smallest line spacing it can from all the applicable font-size settings.
The reason it cannot follow just the line number or brace-matching height is because a line can be a mix of various styles:
b0c86279-d5c6-4983-b610-745c7b7f0e8f-image.png
In the ExtSettings plugin, you can use negative values (you have to use the down arrows to go negative; it wouldn’t let me type a negative value) in the line-spacing entries, which allows you to “squish” the line size lower than Notepad++ wants to go; that might be sufficient for you:
728295fd-5b57-4ab4-bad9-660e31d5758a-image.png
Warning: If you go too negative, it will clip normal text:
1fe90f25-230f-44eb-acb3-2781702d2403-image.png