  We measure IRAC colors of extended emission in several fields covering a
range of Galactic latitudes and longitudes outside of star forming regions. We
determine the nature of the Galactic diffuse emission in Spitzer/IRAC images by
combining them with spectroscopic data. We show that PAH features make the
emission in the IRAC 5.8 and 8.0 $\mu$m channels, whereas the 3.3 $\mu$m
feature represents only 20 to 50% of the IRAC 3.6 $\mu$m channel. A NIR
continuum is necessary to account for IRAC 4.5 $\mu$m emission and the
remaining fraction of the IRAC 3.6 $\mu$m emission. This continuum cannot be
accounted by scattered light. It represents 9% of the total power absorbed by
PAHs and 120% of the interstellar UV photon flux. The 3.3 $\mu$m feature is
observed to vary from field-to-field with respect to the IRAC 8.0 $\mu$m
channel. The continuum and 3.3 $\mu$m feature intensities are not correlated.
We present model calculations which relate our measurements of the PAHs
spectral energy distribution to the particles size and ionization state. Cation
and neutral PAHs emission properties are inferred empirically from
\object{NGC7023} observations. PAHs caracteristics are best constrained in a
line of sight towards the inner Galaxy, dominated by the Cold Neutral Medium
phase : we find that the PAH cation fraction is about 50% and that their mean
size is about 60 carbon atoms. A significant field-to-field dispersion in the
PAH mean size, from 40 to 80 carbon atoms, is necessary to account for the
observed variations in the 3.3 $\mu$m feature intensity relative to the IRAC
8.0 $\mu$m flux. However, one cannot be secure about the feature interpretation
as long as the continuum origin remains unclear.
