How do I wear tzitzit out with a tucked-in shirt? Do I need especially long tzitzit or some other trick?
People normally wear their tzitzis out on the sides. Dress shirts are typically shorter there than in the front and back. It should be possible to use normal-length tzitzis and a normal-sized shirt together.
From personal experience:
I use a Chzon-Ish size begged - about 120cm x 60 cm.
Leave the Tzitzis hanging out where you want them - and then tuck in your shirt around them. The front and back will be tucked in properly, the sides slightly less so.
BTW: According to the Tur (as quoted by the Aruch HaShulchan 8:10) you are not Yotzei the Mitzva of Tzitzis unless you have the strings on all your 4 sides; 2 in front and 2 behind. He learns this out from the Pasuk
The Shulchan Aruch in 8:4 similarly says: