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

The above numbers should be considered as upper limits built into the software for those quantities on an individual basis. In practice, the actual maximums the program can handle are determined by the hardware resources as well as the limits imposed by the operating system. For example, it is highly improbable that a single model with 999,999 members and 99,999 load cases can be solved.

The memory demand of the program is determined by the combined effect of two or more of these terms. For example, when a steel design is performed, the memory required depends on the product of the members being designed (NMD) as well as the number of load cases being designed for (NL). That is, NMD ×NL. So the smaller the NMD, the larger the NL capacity and vice versa.

Also, 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 20,000 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.