Transaction size limit across a single path in a Lightning Network and fee earned
The maximum size for HTLC's are currently defined by max_htlc_value_in_flight_msat and is roughly 42mBTC. The max_concurrent_htlc that you mention seems to be the max_accepted_htlcs and states how many HTLCs can be within a commitment transaction at the same time. This has nothing to do with the maximum payment size (not transaction size) of a single path. Because the single path would only set up 1 HTLC anyway. Thus this is bound by max_htlc_value_in_flight_msat. 

As for the fees this is hard to say because it depends on who pays whom and what routes are being chosen. Assuming a uniform distribution of payment pairs and a flow of payments along the cheapest path the expected earned routing fees are proportional to the betweeness centrality of a node computed on the fee graph. Note however due to the fee rate the fees also depend on the payment size thus the betweenness centrality is almost unexpressable.