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
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.