How to determine bond length and angle in a conjugated pi-system (polymethine)? I have a polymethine molecule with let's say 9 $\ce{C}$ atoms.

$\ce{H-CH=CH-CH=CH-CH=CH-CH=CH-CH3}$

How can I calculate the length of the whole molecule?

I need this number to compute the wavelengths the molecule will absorb (model of linear potential well). 

So what are the average bond lengths and bond angles in this molecule and how are they calculated?

EDIT: Another example for polymethine (just the red part of it):
So what are the average bond lengths and bond angles in this molecule
  and how are they calculated?


1,3-butadiene should serve as a reasonable model for your work.



We can find the bond lengths in the literature.  The carbon-carbon double bond length is 1.338 &angst;, typical for a double bond (reference).  The carbon-carbon single bond length is 1.454 &angst;, shorter than expected due to resonance.  All of the carbons are $\ce{sp^2}$ hybridized, which means that all of the bond angles should be ~120°.

Using this information and vector addition you can determine the length of any polymethine.