Unfortunately, it seems style 'top' doesn't work for the check control. You can fake it by beginning the check label like:
//did -ra dialogname check# $crlf $+ &Visible label
This is only practical if the visible text is split between 2 lines, and prevents the checkbox from being very close to another control above it.
It's also possible that mIRC gives itself access to things it doesn't give us, but I do see the wrapping you're talking about, and it's in quite a few places, such as mirc-options/display has several. Similar to what the other guy suggested above, a slightly better choice is to have the text for the checkbox be $null and have the entire wrap text be in a text control. You'll have to make the text control be very thin to keep the text from getting wiped when the checkbox is toggled. This example works for me without 'option dbu' enabled:
check "", 17, 19 120 10 14
text "&Adjust windows to Toolbar and Statusbar", 917, 35 120 140 26
There are additional styles that could easily be enabled but aren't. For example, BUTTON always displays the text centered, when there are cases where I wish it would make it easy to have the text left/right justified.