They could only give rough guesstimates of Pluto's size. They knew its apparent magnitude and orbital distance, and thus its absolute magnitude, but figuring out a body's size from that also requires you to know its albedo - the reflectance of its surface. Turns out Pluto is quite a bit brighter, and thus smaller, than what was originally assumed.
Also, the search for Pluto was motivated by some apparent irregularities in Uranus's orbit, similar to the ones that led to the discovery of Neptune almost a century earlier. Their magnitude imposed a lower bound on the mass of the hypothetized source of disturbance. However, turns out that these anomalies were simply observational errors and Pluto was neither sufficient nor necessary to explain them.
Good estimates on Pluto's size were only possible after Kharon was found in 1978 which allowed us to calculate Pluto's mass.
Also, the search for Pluto was motivated by some apparent irregularities in Uranus's orbit, similar to the ones that led to the discovery of Neptune almost a century earlier. Their magnitude imposed a lower bound on the mass of the hypothetized source of disturbance. However, turns out that these anomalies were simply observational errors and Pluto was neither sufficient nor necessary to explain them.
Good estimates on Pluto's size were only possible after Kharon was found in 1978 which allowed us to calculate Pluto's mass.