Without being specific about dimensions - I’d say there is plenty room on the top bunk for a large suitcase - it’s where I put mine each time. The bigger issue is how you manoeuvre it down the corridor, through the door and up from the standing space onto that bunk.
Even as someone relatively agile, and with a smallish wheelie case, I’m conscious of all the corridors, doors and walls I or it collide with in order to get there. It’s not generally an ideal space for extremely large or extremely heavy items, I’d say, the guards van (assuming not Fort William) may give a better experience.
Generally in the customer service desert that can be UK rail - I’ve found all the sleeper staff I’ve dealt with very available, kind, friendly and helpful, and they have always sought to find solutions to whatever minor problem I have, so I think you’ll be good on that front.