To help you select the right MIMIC size, here are some quick tips.
The size of the simulation is determined by how many unique devices you want to simulate. This usually corresponds to the number of distinct physical boxes you have on your network, eg. each separate box is one agent. In general, to get an approximate size, count the number of boxes. Particularly, for a router with many interfaces, you would still only need one agent, unless the router has community-string indexed sub-agents each returning different management information (and you want to simulate this difference).
The sizes listed in the checklist correspond to the licensed sizes in MIMIC. You would want a size at least as large as the network size you want to simulate. Notice though, that you can start with a smaller size, and easily upgrade to a larger size later (pay-as-you-grow).