For several months the size of the UTXOs set has been exploding with almost 20MB more every day. So I would like to know from what size it may be a problem for small nodes such as Raspberry Pi 4 for example?
Bitcoin Core adds outputs to the UTXO cache either when they are newly created, or when they’re spent by inputs in transactions the node learns about. Whenever the node’s dbcache is full, it flushes the entire cache and starts over. A larger UTXO set means that a smaller portion of them can fit into the cache before the cache gets flushed. This especially slows down IBD, but may also slow down node operation at some point.
I would expect that node performance would degrade gradually rather than at some specific point.