Question: Do I need to wash on Shabbos day If I don’t want to? It’s too heavy etc….
Answer: Eating Challa by the Shabbos meals is of primary importance and should be practiced, unless there is a medical consideration. The required amount per meal is only slightly more than the volume of an egg (see note) and thus a healthy person shouldn’t feel too heavy from eating just that amount.
If there is a concern that one might not feel well (aside for usual tiredness) after eating this minimal amount, or it would cause pain, there would then be place to advise an alternative to eating an K’beitza of Challah, as there may be a medical consideration to take into account. In this case, one can suffice with eating only one Kazayis of bread. (Shulchan Aruch Admor Hazaken Orach Chayim 274:6.)
Additionally, there is the view of the Maharil (the Alter Rebbe’s brother) who testified that towards the end of his life the Alter Rebbe changed his earlier ruling and said that one fulfills the obligation of a “Seudah” for Shabbos also with Mezonos. This would only apply to baked Mezonos foods (לחם שאינו גמור), foods that when affixing a meal on it and eating the requisite amount would acquire the Bracha of Hamotzi. However this should only be relied on in cases of great need, not under ordinary circumstances.