There are several problems with broadcasts. First, unlike not flooded unicast or IGMP controlled multicast, every broadcasts is transmitted down/out every port (if same broadcast domain). I.e. much LAN traffic, other than broadcast traffic, is never presented to the host's NIC.
Second, NICs are designed to quickly ignore frames of no interest, but with broadcasts, the host must accept and analyze the contents to determine whether it's of interest to the host or not. I.e. undesired broadcast place a much higher burden on the host than other undesired traffic.
The impact is the host's NIC and other system components need to spend their time ignoring undesired broadcasts whose time would be better employed, productively, for other purposes.
As to "size", that shouldn't matter too much beyond taking up buffer space until the host can decide it can discard them. I.e. larger sizes, likely, do not increase resource consumption, again beyond buffer space usage.
Hard to say what your input/out discards represent, but yes, lots of broadcasts might impact one or both.
BTW, don't forget, network devices, with network management access, are also hosts. I.e. a network device's management "port" exposed to broadcasts can impact the device's overall performance.
