What's the smallest flier that migrates?
The marmalade hoverfly (Episyrphus balteatus) flies to Southern Europe and back every year. 


It's difficult to track and find them compared to birds and butterflies, here is a study witch citations: https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1570-7458.2007.00568.x?purchase_referrer=scholar.google.fr&amp;tracking_action=preview_click&amp;r3_referer=wol&amp;show_checkout=1

it is now known that large numbers of E. balteatus migrate South during the autumn period. Therefore, it has been sug-gested that a comparable migration to the North helps tore-establish E. balteatus populations in northern Europe during the spring. There are many records to show that some hoverfly adults are active so early in the spring that they could not possibly be long-distance migrants. Nevertheless, although some adults remain active late into the autumn, few authors have managed to find females in overwintering sites

studies have suggested that E. balteatus migrates in large numbers during late summer and autumn to the Mediterranean area and only return to central and northern Europe during the following year(Aubert et al., 1976; Gatter &amp; Schmid, 1990; Kehlmaier &amp; Martínez de Murguía, 2004; Hondelmann et al., 2005).

https://www.sciencenews.org/article/flying-insects-tell-tales-long-distance-migrations