user105880: 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? 

user81940: 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. 

105880: <p>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? </p>
 81940: <p>The maximum size for HTLC's are currently defined by <code>max_htlc_value_in_flight_msat</code> and is roughly <code>42</code>mBTC. The max_concurrent_htlc that you mention seems to be the <code>max_accepted_htlcs</code> 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 <code>max_htlc_value_in_flight_msat</code>. </p>

<p>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 <a href="https://www.youtube.com/watch?v=L39IvFqTZk8" rel="nofollow noreferrer">expected earned routing fees are proportional to the betweeness centrality of a node computed on the fee graph</a>. Note however due to the fee rate the fees also depend on the payment size thus the betweenness centrality is almost unexpressable. </p>
