At mine, the assessments have involved first a physical examination and then just an assessment like any other MH one really, except with more focus on food and all that. They'll most likely ask about your intake, behaviours, body image, thoughts, and ask what you'd like/might find useful. They'll most likely also ask about sleep, mood, self harm, relationships an any other previous treatment.
Assessments are never the most fun (especially the physical part - but try to remember that they genuinely, honestly so see ALL shapes and sizes and statistics, and if it's a decent place they will hopefully try to tackle more the psychological causes rather than focu on the physical stuff) but try to be honest so that they can help you best. If there's anything else I can answer then I'd be happy to. Good luck x