What is the actual size of a nucleotide?
If you want experimental papers, we should be precise about what we're measuring. "The dimension of a nucleotide" is rather imprecise, as a nucleotide is a rather oblong, knobbly thing. 

0.34 nm is a relevant measurement related to nucleotides, but it's specifically the distance between consecutive bases in a standard B-form DNA helix - that is, what's the spacing between the "rungs" on a DNA ladder.

This measurement can be confirmed by several experiments, primarily those involving X-ray or neutron crystallography or scattering. For example, both the Wilkins and Franklin papers that were published back-to-back with the famous Watson &amp; Crick DNA structure paper referrence the strong 0.34 nm reflections from X-ray fiber diffraction which Watson &amp; Crick reference as the spacing of the bases in their model of DNA structure. Other papers have confirmed the measurement. This 1983 Biomed Biochim Acta paper uses wide-angle X-ray scattering to find it. Additionally, there are a large number of structures in the Protein Databank which contain DNA structures - these have been determined by a number of different approaches (X-ray crystallography, neutron diffraction, NMR, electron microscopy, etc.) and all of which are consistent with the 0.34 nm average spacing of base pairs. (Modulo minor structure-to-structure variations.)