What is the maximum model size that STAAD.Pro can handle ?

The limits are as described below. The details are provided in technical reference manual section 5.2 titled Problem Initiation and Model Title

1.Joint number 1 to 999999
2.Number of joints: 200000
3.Member/Element numbers: 1 to 999999
4.Number of Members, Plates, and Solids: 225000
5.Load Case numbers: 1 to 99999
6.Number of primary and combination cases: 10101
7.Number of modes and frequencies: 2700
8.Number of load cases that may be combined by a Repeat Load or Load Combination command: 550

However operating systems impose limits on file and memory usage by applications and such limits can be reached with smaller models specially those with large number of load cases. So for all practical purposes,  one should try to keep models within 30,000 members/plates. Bigger models with large number of load cases not only take large time to analyze but working with such models can be inconvenient too as simple operations like navigating between the pages, that require refreshing the model view, can be slow.