Would appreciate suggestions for hotels in or near city center. Prefer hotels smaller than 30 rooms, with restaurant, under 150 euros/night, and some english spoken (not mandatory), private toilets with showers.

A cheap, simple option in the city center is Pension Lerner. It is well below the budget you suggest, so you may well want to spend more for additional charm, but it fulfills all your stated requirements except no restaurant. It does serve a large breakfast though. Here is how I described it before on this forum:

Pension Lerner is in the historic center of Vienna, walkable to everything, and easily within your price range. (According to their website 80 to 88 euros for a double in high season.) I stayed there in May 2004. It is simple, but pleasant, with a big breakfast included. While the decorating isn't charming so much as clean and homey, our room had a fair amount of space for a European hotel room, and nice big windows that got a lot of sunlight and looked out into the fairly quiet street. Horsedrawn carriages would clop by, which was pleasant. See www.pensionlerner.com.

I have stayed in two hotels in Vienna.
They were the Alt Wienerhof near the Westbahnhof, and the Carlton Opera, which is a little southeast of the Ring.

Of the two, I thought the Carlton Opera was slightly better. The breakfast in both of them was excellent with many choices of all kinds of foods.

The bathrooms were the most American style I have seen in Europe.

There is a gourmet (expensive) restaurant associated with the Alt Wienerhof.

Both are close to transportation.

A double sized bed in both hotels was the equal of our king bed.
It was more like two singles shoved together.

Depending on what time of year you will be there, the Carlton Opera had one huge plus: It is air conditioned. That is important because street noise often can be obtrusive. With the air on, you can keep the double windows shut and the noise supressed.

Were I go to Vienna next week, I would try for a room at the Carlton Opera.
There may well be better places, but this one I know.

Try the Fuerstenhof:
http://www.hotel-fuerstenhof.at/index2.htm
Though it is a little bit out of city center, it is a great place to stay and with the U-bahn station just in the square in front of the place you can be in city center in under 10 min.

The Fuerstenhof is only 3 U-Bahn stops to the Ring, and another two stops to Stephansplatz. Its a great location for those who are planning daytrips out of town by train. The main train station (Westbahnhof) is adjacent to the hotel, so only a few minutes walk.

We stayed at "the Rooms" outside the city center, but near the station. Lovely room, great breakfast and around 100 euro's.

Marilee, was the hotel in Vienna named "The Rooms"?

Yes, it is called "the rooms bed and breakfast vienna". It is a short walk to the station, but is out of the center of town. It didn't bother us at all (we are in our 50's), the walk to the station was 5 minutes, very easy. The price is right!