My wife and I will be spending a couple of days in Budapest before taking a Danube cruise in September. We are considering going to the opera one night (The Barber of Seville is scheduled.) A couple of questions:
1. Dress for the Opera - Will I need a sport coat/suit to attend, or is more casual clothing acceptable?
2. Seats - We haven't yet purchased seats. Are even the cheap seats of reasonable size? We are not small people, and I don't want to purchase seats we won't fit in.
If not the opera, is there anything else that might be recommended for evening entertainment in Budapest?

I would already consider a "sportscoat" to be rather casual. And regarding seats: I now depends a bit whether you guys are vertically or horizontally big. First row might give you more legroom, however seat width should be always the same.

Budapest has a nice muscial theatre where you can experience broadway blockbusters in Hungarian language. Quite interesting. Beside that: Tons of very good restaurants in the city and tons of other options depending on your preferences.

Here's the website, where you can book tickets and get the calendar of performances.

The opera in Budapest is a great experience, and usually quite economical as well.
If you have not purchased tickets yet- you may want to check with the conceirge at your hotel to see what the ticket costs are through them- although the price is usually more, or has a service charge of around ~15% IIRC- the benefit is that they normally have very good seats.

In terms of dress- I think a sports coat is a good touch- a lot of people dress up for the opera in Budapest- however it is not absolutely required. I wore a suit without a tie, and it was fine.

Barber of Seville is a great opera also- enjoy your time there! If the weather is nice, I would also go upstairs (well- depending on where you are sitting it may be the same level) to the bar with the terrace- it is beautiful to have a quick coffee or drink and admire the building from this terrace level.

Consider getting a box, otherwise general seating should be fine along the sides.

Lots of good restaurants close by.

Subway may close earlier than you expect. We felt safe walking late at night, but it was a much longer walk than we expected!

Even if you decide not to see an opera, the tour of the opera house was fantastic.

A good general rule for box seating in ALL opera houses is to only purchase tickets in the first row of the box. Not only are sight lines often difficult if you are not in front, the low ceiling tends to quash the sound, resulting in difficulty in properly hearing the music.

Another tip - don't avoid performances of indigenous works (for example, Bela Bartok's "A k�kszak�ll� herceg v�ra" (Duke Bluebeard's Castle), or the Hungarian folk opera "H�ry J�nos"). Not only do the singers and orchestra have the music in their souls, national pride often means good casting and that care will be taken that the production makes the best case for the work in question.