What is the smallest water plant?
What is the biggest water plant?

Dear dong86-ga 

Largest water plant -
The largest water plant is the Giant Water Lily (Victoria amazonica)
originally found in the Amazon basin in
 in South America. They can grow leaves up to 8ft across and can bear
the weight of a small child.

Source: 
Queen’s University Ontario Canada
http://educ.queensu.ca/~science/main/profdev/CMSTTT2.htm
Confirmed at:
http://www.mkcompanies.com/StudyGuides%5C2288-atm.pdf
and:
http://216.239.39.100/search?q=cache:N58OYuPrqhEC:www.sarasota.k12.fl.us/seatrek/curric/msbg_unit2.pdf+%22largest+water+plant%22&hl=en&ie=UTF-8

An image is available at:
http://www.amazonian-fish.co.uk/c6b1.html
For more images use the image search strategy below.

Smallest water plant -
“The world's smallest plant is water meal (Wolffia globosa), a
floating aquatic herb which, when mature, is not much larger than the
period at the end of this sentence.”

Brooklyn Botanical Garden
http://www.bbg.org/gar2/topics/botany/parts_grouping.html
Confirmed at:
Wild California
http://www.calacademy.org/calwild/summer2000/html/horizons.html

An image is available at:
http://waynesword.palomar.edu/imgwogl.htm
For more images use the image search strategy below.

Please ask for clarification of any part of this research, or if the
links do not work, before rating the answer.

answerfinder-ga


Web search>
"largest water plant"
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22largest+water+plant%22&btnG=Google+Search

smallest "water plant" "in the world"
://www.google.com/search?q=smallest+%22water+plant%22+%22in+the+world%22&btnG=Google+Search&hl=en&lr=&ie=UTF-8&oe=UTF-8

Image search>
giant water lily
http://images.google.com/images?q=giant+water+lily&hl=en&lr=&ie=UTF-8&oe=UTF-8

Wolffia globosa
http://images.google.com/images?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=Wolffia+globosa&btnG=Google+Search

There are a myriad of unicellular algae, all considerably smaller than
the period at the end of this sentence.  I would search among the
cyanophyta, the blue-green algae, to find the smallest.