I found it in the meantime:

Using the  length_of function to check an arcminute length, a meridian, the equator and pole diameter:

        
    from skyfield.api import Topos, load
    from skyfield.functions import length_of
    
    ts = load.timescale(builtin=True)
    t = ts.utc(2021, 1, 1)
    
    b1 = Topos(0., 0., elevation_m=0.0)
    b2 = Topos(1. / 60., 0., elevation_m=0.0)
    print(round(length_of(b1.at(t).position.km - b2.at(t).position.km), 5))
    
    b3 = Topos(90., 0., elevation_m=0.0)
    b2 = Topos(90.0 - 1. / 60., 0., elevation_m=0.0)
    print(round(length_of(b3.at(t).position.km - b2.at(t).position.km), 5))
    
    b2 = Topos(0., 1. / 60., elevation_m=0.0)
    print(round(length_of(b1.at(t).position.km - b2.at(t).position.km), 5))
    
    b4 = Topos(90., 0., elevation_m=0.0)
    print(round(length_of(b1.at(t).position.km - b4.at(t).position.km), 3))
    
    b5 = Topos(0., 180., elevation_m=0.0)
    print(round(length_of(b1.at(t).position.km - b5.at(t).position.km), 3))
    
    b6 = Topos(-90., 0., elevation_m=0.0)
    print(round(length_of(b4.at(t).position.km - b6.at(t).position.km), 3))
    
    #Meridianminute der geographischen Breite am Äquator 1842,90 m,
    #an den Polen aber 1861,57 m
    # Bogenminute am Äquator eine Bogenlänge von 1855,31 m.
    #Meridians vom Äquator bis zum Pol von ca. 10.001,966 km
    #Äquatordurchmesser* 	12.756,27 km
    #Poldurchmesser* 	12.713,50 km
    


The results are very precise:

   
> 1.8429 km 
> 1.86157 km 
> 1.85532 km 
> 9004.939 km 
> 12756.273 km 
> 12713.504 km

Of course the meridian is measured thru the ground and not at the surface, therefore 9004.939 instead of 10,001.966 km