What is the exact length of "viersta" (i'm not sure about the
spelling), an old Russian measure

I believe you're referring to the Russian "versta", also known as
"verst" or "werst."

"versta [Russian.] 

(Plural, vėrsty.) In Russia, a unit of length, = 500 sazheni, about
1.067 kilometers (about 0.66288 mile). Also romanized as werst and
verst."

Sizes.com
http://www.sizes.com/units/verst.htm

"VERST
 Pronunciation: vurst...

 Definition:   [n]  a Russian unit of length (1.067 km)...  
  
 Definition:   \Verst\, n. [Russ. versta: cf. F. verste.]
 A Russian measure of length containing 3,500 English feet.
 [Written also {werst}.]"

Hyperdictionary
http://www.hyperdictionary.com/dictionary/verst

Google search strategy:

Google Web Search: verst OR versta
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=verst+OR+versta

If this is not the correct term, or if anything is unclear, please
request clarification. I'll gladly offer further assistance before you
rate my answer.

Best regards,
pinkfreud