An odd quirk of modern RDP is that it seems to still “prefer” standard display sizes (WXGA, etc.). At least when connecting to Windows servers, if the RoyalTSx window isn’t exactly a common display size, there’ll be letterboxing around the session. Once connected, you can resize the window a smidge or reconnect the session and RDP will adapt to whatever window size you have, but that initial connect having letterboxing is annoying, especially if you’ve just connected to a whole folder’s worth of sessions.
Maybe offer the Terminal-like ability to resize the window in “whole” increments while a key is held? Or add a menu option “resize connection area to…” with an editable list of common resolution options?