Hi Ricky 
Welcome aboard Warships @ Armorama 

We still have some time before we start DDD2, and I can suggest some good models - not that tinny 

Fujimi 1/200 I-19 IJN submarine
Fujimi 1/200 Type IX U-boats (three versions)
Althought these models are from the seventies, they are good models and relatively easy to find
ICM 1/144 Type XXIII U-boat 
https://armorama.kitmaker.net/review/815 Revell 1/144 Type XXI U-boat
Revell 1/144 Type 206 U-boat (Modern) 
Revell 1/144 Type 212 U-boat (Modern)
Revell 1/144 Wilhelm Bauer (Mod Type XXI)
Revell 1/125 Type VII-B U-boat
Revell 1/220 USS Lyonfish (or any of the reboxed names) Gato class boat
Trumpeter 1/144 Kilo
Trumpeter 1/144 Romeo (Type 33)
Trumpeter 1/144 Type 33 G (missile laucher mod)
Trumpeter 1/144 Seawolf
All these models are great and they will make an impressive display without being the "monster" Revell Type VII-C
You can go to DDD Campaign gallery to see what we achieved on the first submarine Campaign 
 http://photos.kitmaker.net/showgallery.php/cat/13882
http://photos.kitmaker.net/showgallery.php/cat/13882Some of the models stated above are there, so ... just choose one
Hope this helped
Skipper