The formation of protoplanetary discs during the collapse of molecular dense cores is significantly influenced by angular momentum transport, notably by the magnetic torque. In turn, the evolution of the magnetic field is determined by dynamical processes and non-ideal MHD effects such as ambipolar diffusion. Considering simple relations between various timescales characteristic of the magnetized collapse, we derive an expression for the early disc radius, $ r simeq 18 , {rm AU} , left({eta_{rm AD} / 0.1 , {rm s}} right)^{2/9} left({B_z / 0.1, {rm G}} right) ^{-4/9} left({M / 0.1 msol} right) ^{1/3},$ where $M$ is the total disc plus protostar mass, $eta_mathrm{AD}$ is the ambipolar diffusion coefficient and $B_z$ is the magnetic field in the inner part of the core. This is about significantly smaller than the discs that would form if angular momentum was conserved. The analytical predictions are confronted against a large sample of 3D, non-ideal MHD collapse calculations covering variations of a factor 100 in core mass, a factor 10 in the level of turbulence, a factor 5 in rotation, and magnetic mass-to-flux over critical mass-to-flux ratios 2 and 5. The disc radius estimates are found to agree with the numerical simulations within less than a factor 2. A striking prediction of our analysis is the weak dependence of circumstellar disc radii upon the various relevant quantities, suggesting weak variations among class-0 disc sizes. In some cases, we note the onset of large spiral arms beyond this radius.
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