Learn Arabic (Modern Standard): هل البيت واسع؟

This is how Arabic forms a basic nominal sentence (an "is" sentence). The subject, البيت (the house), is definite, while the predicate describing it, واسع (spacious), is left indefinite. This contrast tells us it means "The house is spacious" rather than "the spacious house" (which would be البيت الواسع).

How exactly does the indefinite ending (Tanween) sound on واسع?

With full formal pronunciation, it is pronounced as "waasi'un". That "un" sound at the end (Tanween) is the marker of an indefinite noun or adjective. Because it is the predicate describing the house, it takes this basic indefinite ending.

Does the question word هل change the word order of the sentence?

No, it doesn't change anything else! In Arabic, to turn a simple statement like البيت واسع (The house is spacious) into a yes/no question, you simply put هل at the very beginning.

If I wanted to ask about a feminine word, like "the room" (الغرفة), would واسع stay the same?

No, the predicate must match the gender of the subject. Since البيت is masculine, واسع is masculine. If you asked about the room, you would need to add the feminine ending ة (Ta' Marbuta) to make it هل الغرفة واسعة؟.