I've looked into this before...
Already mentioned is the Hansen Roberts (2 ⅜” by 4”)
and the Zerner-Farber Tarot Deck (2 ¼” by 4”).
I read with a
standard 2 ⅜” by 3 ⅜” poker deck, the
same exact dimensions of the African Tarot. In that
range is the Tarot Nova at 2 ¾” by 3 ¼” (which was
previously mentioned in the posts above), and the US
Games pocket editions at 2 ¼” by 3 ½” (the RWS,
Universal Waite, Crowley Thoth and Goddess).
The dimensions of 2 ⅜” by 4 ⅜” seem to be kind of
popular, (though slightly larger than poker-deck sized);
these are the measurements of the Chinese, Native
American (Gonzalez), Tarot of the Ages, Art Nouveau
(Myers), and the RWS variation by Fronteras (from a
set simply called "Tarot").
Other decks on the small side:
Cagliostro: 2 ⅜” by 4 ¼”
Dragon (Donaldson): 2 ¾” by 4 ⅜”
Medicine Woman: 2 ½” by 4 ½”
Motherpeace “Mini”: 3” diameter
New Orleans Voodoo: 2 ¾” by 4 ¼”
Osho Zen: 2 ⅞” by 4 ½”
Spiral: 2 ¾” by 4 ½”
Ukiyoe: 2 ⅜” by 4 ½”
Wonderland Tarot: 2 ⅜” by 4”
Fair mention must go to the "mini" tarots, which are
perhaps comparable to "patience" playing cards in size.
The "mini" dimensions of 1 ¾” by 3 ⅛” belong to both
the Universal Tarots and Castelli's Tarot Art Nouveau;
there's also the mini Albano-Waite at 1 ⅜” by 2 ⅜”
and the 1 ⅛” by 2 ½” miniature Connolly Tarot.
EDITED for disclaimer...
These notes are from some time ago now,
so they don't take into account newer decks.
Joan Bunning's site consistently lists the card
measurements with her deck reviews:
http://www.learntarot.com/decks1.htm