Transaction size limit across a single path in a Lightning Network and fee earned Given MPP is included in the current version of lnd, an arbitrarily large transaction can be processed in the network. What is the transaction size limit across a single path? Is it still bounded to 0.0429 BTC (limited by max_concurrent_htlc)? What is the estimated fee earned (average as well as maximum) by a lightning node per day?
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.