Lewis & Engelman (1983) Lipid bilayer thickness varies linearly with acyl chain length in fluid phosphatidylcholine vesicles. J. Mol. Biol. 166: 211 - 217.
Table 1 and Figure 3 have the information that you need. For C14:0 the thickness of the hydrocarbon bilayer is given as 23 Å. Just in case this is homework, I'll leave you to convert that to nm.
Just to follow up on the comments, as hinted at by Shigeta, if you wanted to calculate an estimated bilayer thickness you would have to take into account the C-C-C bond angle which for tetrahedral carbon is 109.46°.
Thus the distance between two C atoms in an extended C chain is 2 x 1.5 x sin(109.46/2) = 2.45 Å
For C14:0 the length is then 6.5 x 2.45 = 15.9 Å (from methyl C to carbonyl C).
Therefore, calculated bilayer thickness = 31.8 Å plus any distance between the two C chains in the centre of the bilayer. This is still a lot bigger than the reported 23 Å.
Clearly the bilayer does not consist of extended hydrocarbon chains. The chains are presumably partially collapsed (on average) and may also interdigitate to some extent.