Small vs Large neurons
Depending on the neuron, small/large could mean one of two things:

1: Axon diameter, larger axons have lower resistance, so transmit action potentials faster see tables here. Important for neurons that cover long distances to convey information quickly.

2: What I think you mean though is the size of the cell body, where some neurons, e.g. Purkinje cells are much larger than nearby golgi cells. In which case the size is related to the function, large cells tend to have large dendritic fields and integrate many inputs into a single output, often relayed to a different area of the brain. Small cells are typically interneurons that only have an effect over a very small area.