Use 'infinitesimal' to emphasize the extreme smallness of something compared to what's expected. With his savings being infinitesimal, buying a new car seemed impossible.
Pro Tip 2/3
Figurative Use
'Infinitesimal' can be used figuratively to express something negligible or insignificant. Her chances of winning are infinitesimal, but she remains hopeful.
Pro Tip 3/3
Not Hyperbole
Avoid using 'infinitesimal' as a hyperbole for 'small'; it means something much smaller. The artist added an infinitesimal dot of red to the painting, almost unseen.