Minimum Packet size for a 100Mbps connection in ppp

Hi all,
My Mikrotik CCR1036 working with 500 Mbps bandwidth is total and about 250 pppoe customers. each users plan start from 2 Mbps to 100 Mbps. But some times facing issues in downloading.
While i checked pppoe traffic status ,saw heavy packet drops in some pppoe connections. I want to know what is the minimum packet size to be set in “default-queue” for, currently default is set to 10 packets. Anyone know about how to set packet and maximum or minimum to set?