Co-precipitation particle size
Kinetic aspects:
Low concentrations of interacting ions decrease rate of precipitation, leading to bigger crystals.
A high ionic strength of spectator salts leads to lower activity coeeficients , and  decreased rate of precipitation, leading to bigger crystals.
Higher viscosity leads to lower collision rates, decreased rate of precipitation, and to bigger crystals.
Low temperaturee leads to bigger crystals, as rate of collisions leading to crystallisation is lower due higher viscosity, lower molecular speed and lower rates of interactions leading to precipitation.
Note that kinetic aspects may be hard to involve in the proces control, aside of concentrations.
Thermodynamic aspect:
Crystals of low sizes have high surface Gibbs energy and therefore bigger crystals are thermodynamically preferred.
This is initially overrun by the kinetics of the precipitation, but in long term there is ongoing recrystallization in favour of bigger ones. This can be extremely slow for very little soluble salts.
This process is speeded up by high temperature, what increases rates of the opposite processes of dissolution and crystallization. AFAIK, the gravimetric procedure to determinate sulphates as barium sulphate involved heating $\ce{BaSO4}$ suspension near below boiling point for I guess 1-2 hours to speed up recrystallization. As bigger crystals are easier to manage, espacially in this particularly case, as $\ce{BaSO4}$ is known to form too fine crystals.