Hi Eule,
on (most of LH's) 737-500 the window seat in the exit row offers you most leg room since there is no seat in front of you.
On the 737-300 you are referring to the exit row does not really offer you the extra leg room, one would expect in an exit row.
From your first post I gather, that you are not at all familiar with LH's narrow bodies... well, for intra-european flights (not for domestic flight though) the seats in Business Class are being converted from 6 abreast to five abreast. Due to emergency and evacuation restrictions the first row in economy left hand side (in flight direction) is always Business Configuration, which means, it has a wider A and C seat, but not B seat at all. The only exception from that rule is the A321 when the cabin divider goes between rows 8 and 11. In that configuration row 11 is 6 abreast instead of 5 abreast.
Most passengers find the room and privacy on these two particular seats quite nice and choose these seats rather than the exit.
But, and there is almost always a "But"
these seats go early and fast and I have no idea if they are available through telephone check-in. Nevertheless I'd give it a try.
Anyway, enjoy your trip!
Happy Landings!
I Love LH
------------------
Providing good service also needs customers willing to accept and appreciate good service.