Hello,
First, I don’t think you’ll be able to get anything interesting by averaging epochs of 60s together: after a few seconds, the signals are not locked to you stimuli anymore, and there is very little chance that it would average across trials brain signals that are aligned in time.
Averaging over time is not a good solution either. This is detailed in other forum posts, like this one: Unusual values for statistics on Scouts: Constrained vs Unconstrained
The reason for which you get this error is independent from these questions. It occurs because you don’t have enough memory available to reconstruct the full source files in memory. The screen capture you posted shows files saved in the optimized way (imaging kernel + recordings: http://neuroimage.usc.edu/brainstorm/Tutorials/SourceEstimation#Computing_sources_for_single_trials). The sources are kept in this format for the interactive display, but any non-linear operation applied on the file will first rebuild the full source matrix (Nsources x Ntime) before. This operation crashes because you don’t have enough memory.
To check the size of the largest variable your computer can handle, go to the Brainstorm preferences (File > Edit preferences), and see what you have at the bottom of the window.
Try with shorter time windows or with less vertices in your source maps (to check the dimensions of the variables saved in a file: right-click on the files > File > View file contents)
Francois