SanDisk MobileMate USB 3.0 microSD Card Reader is the smallest microSD Card Reader for USB-A ports that I have came across.

It measures 31 x 21 x 9 mm and weighs just 3.4g.

SanDisk MobileMate USB 3.0 microSD Card Reader - Packaging Front
SanDisk MobileMate USB 3.0 microSD Card Reader – Packaging Front