You should actually be asking what the largest and smallest
trimmed games are. There are at least 10 ROMs with an untrimmed size of 512MB:
Nanami no Oshiete English DS - Mezase TOIEC Master (378MB trimmed)
Love Plus Plus (387MB trimmed)
Inazuma Eleven 3 - Sekai e no Chousen! Bomber (407MB trimmed)
Inazuma Eleven 3 - Sekai e no Chousen! Spark (412MB trimmed)
Inazuma Eleven 3 - Sekai e no Chousen! The Ogre (447MB trimmed)
Ninokuni - Shikkoku no Madoushi (459MB trimmed)
Tokimeki Memorial - Girl's Side 3rd Story (498MB trimmed)
Hakuouki DS (498MB trimmed)
Hakuouki - Zuisouroku DS (512MB trimmed)
Eigo de Tabisuru - Little Charo (512MB trimmed)
So
Hakuouki - Zuisouroku DS and
Eigo de Tabisuru - Little Charo are tied for the largest ROMs.
Ninokuni is only the 4th-largest trimmed ROM. There may be more, but I haven't kept track for almost a year now.
As for the smallest ROMs, there are far too many to list those with an untrimmed size of 4MB. But the smallest
trimmed ROMs are ≤1MB in size. There are three that I know of:
Unou Kaiten - Match-Bou Puzzle DS
La Culture Generale pour les Nuls
Billiard Action
*EDIT 1*
I guess I posted exactly the same thing Cyan said, except I included trimmed sizes
And for the record, there are
no 2GB DS ROMs.
*EDIT 2*
The smallest ROM is
XXXX - Dr. Kawashima's Brain Training - How Old Is Your Brain? Sudoku Download 2 (WiFi Demo), which is 328KB.