‘Grande’ before the noun
 

- Londres y París son dos ciudades grandes:
London and Paris are two big cities. (size)

- Londres y París son dos grandes ciudades:
London and Paris are two excellent cities.
(quality e.g., great museums, theatres, shopping centres etc.)

Are my interpretations of 'grande' and its position in the sentences correct?
Could the second sentence possibly refer to 'size', too?

Many thanks in advance.

"Grandes ciudades" may have both meanings, depending on the context and the rest of the conversation. But yes, generally, the position changes the nuance. :)

Thank you so much!