Physical size in different redshift
You're correct, except that you should multiply, not divide, by 3600 (you're converting from degrees to arcsec, not the other way round). You get $5.7\times10^{-5}{''}$, but the correct result is 36002 times higher, i.e. $738.6''\!\!\!$.
But why not just let astropy do the job:
z     = 0.3
x     = 3.29 * u.Mpc
theta = x * cosmo.arcsec_per_kpc_proper(z)

print(theta.to(u.arcsec))
---&gt; &lt;Quantity 738.61092463 arcsec&gt;