Reference: MsJ's article
http://jpicforum.info/showthread.php?t=819
Here's a bit of a History Lesson.
Waka translates to "Japanese Poem". It isn't actually a form, more like a category. Tanka, Haiku, Choka, and others are forms of Waka. Waka was written in 5-7-5-7-7 because that was the only form the Japanese had. Most of their poetic influence was Chinese, and so they used Chinese forms.
Japanese poet and critic Masaoka Shiki created the term "Tanka" in the early twentieth century for his statement that "
Waka should be renewed and modernized". Until then, poems of this nature had been referred to as waka or simply uta ("song, poem, verse"). Haiku is also a term of his invention, used for his revision of the old hokku form, with the same idea.
So...if you see a poem labelled as Waka, and it isn't 5-7-5-7-7, well, it may still be Waka.
Thanks, MsJ!