= Solution
For a <concentration blob> with $g$ ideal segments, $\xi\sim a\sqrt g$ and concentration matching gives $\phi\sim ga^3/\xi^3\sim g^{-1/2}$. Thus the <concentration-blob size in a theta solvent> is
$$
\boxed{\xi\sim\frac a\phi=6.7\,\mathrm{nm},\qquad g\sim\phi^{-2}=100.}
$$
The whole chain contains about ten blobs, so the separation of scales is moderate. Below $\xi$, theta-solvent segments have <ideal chain> statistics; above it, screening also gives Gaussian random-walk statistics for the sequence of blobs. There is no good-solvent self-avoiding regime to insert in this particular theta-solvent calculation. Density correlations and segment dynamics can still cross over at the blob scale.
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