Making function nodes is very easy, you just add BlueprintCallable and BlueprintPure specifiers UFUNCTION and also add Category=Something specifier and C++ function turn magically in to blueprint node, here you got tutorial:
Note: you don’t need to use FriendlyName and CompactNodeTitle so obsessively as this tutorial use, UHT will pick name of function
The limits is what reflection system supports, most notably there only 2 integer types supported int32 and uint8