Good morning, Francesco;

Need some specifications here. This question is remarkably subjective and will vary widely based on what is meant by "small" and what is meant by "powerful." If we are clearly limited to electronic flash and cannot use the older flashbulbs, then we are really out in an area where the specific numbers for any one of the qualities will probably limit the numbers for the other qualities. My Minolta Auto 25 will easily go into my shirt pocket, but the Metz 60 CT1 will outperform the Auto 25 by a factor of about 4 or maybe 5.

From your description of the intended use, it sounds like you are just opening the shutter in a dark area, and using multiple flashes from the electronic flash to illuminate the scene to be recorded onto the film. If you are using something that can be carried in your shirt pocket, you will need to space the flash positions closer together and make more of them than you would with something carried on a shoulder strap or in a bag.

The best general suggestion I can offer is to just take what you have, and start experimenting and documenting what you did for each identified exposure. Then choose the one you like the best for that scene and situation. One thing to be said for a smaller, less powerful unit in this case, is that you will probably have a more even illumination from that unit with more closely spaced together flash positions than you probably will with a more powerful unit with the flash positions spaced farther apart.

Try what you have and see if you like what you get.