1) Am I correct to assume that the larger page size some user session has set - the larger portion of cluster/coordinator node resources will be hogged by the corresponding session?
2) Do I understand correctly that page size (imagine we have no timeout settings) is limited by RAM and iops which I want to hand down to a single user session?
Yes for both of the above. More rows will be pulled into memory simultaneously with a larger page size, thus using more memory and IO.
3) Am I correct to assume that the page size/read request timeout allowance I set is direct representation of chance to lock some node to single user's requests?
Concurrent reads can occur on a node, so it shouldn't "lock" the node to a single users request. However you can overload the node, which may be effectively the same thing. Don't set page sizes too high, otherwise the coordinator of the query will end up doing a lot of GC.