Screens run the course from the cheap companies (Focupix/Elite) to typical mid-level (Carada, Da-Lite, Draper), to the top shelf (Stewart) and buying an appropriate screen is always a question mark.
It's important to note that a good screen will be matched to the room and the house and will not change. Like a really good set of speakers, you don't replace a screen every few years like you may an A/V receiver or a projector (!) so spending more on a screen is not at all inappropriate if your budget and enthusiasm allows.
Fixed framed screens are the best bang for the buck product on the market. They basically are hung on the wall like a picture with a very large frame and screen material stretched tight across that frame like a canvas.
The big plus: A perfectly flat viewing surface up against a wall which really isn't disturbed by breezes, etc.
The big down: A large flat white screen which is always visible and can't be hidden away to protect it.
Good model for the HD20?
Almost all screen manufacturers make this one! Get one with a velvet lined frame and proper viewing surface for the room (let's go with white here!)...
Focupix delivers 100" for about $300 or so (they are currently out of stock)
Carada delivers 100" for about $800
DaLite and Draper run around $1,000 or more.
Stewart... a fair bit more.
Manual roll up screens are a great entry point but are the second worst screen possible on the market. They all end up getting waves in the material whether left down or rolled up regularly and only a few manage to hold their flatness for more than a few months.
Big plus: The cheapest prefab screen available!
Big minus: Waves in the material means you should plan to replace it regularly or live with the waves.
About $100 to $200 from Focupix, Draper, and DaLite.
The HIGH POWER from DaLite is well regarded for a manual screen surface and is a bit more than other screen surfaces, but will last a bit longer for the money...
http://www.projectorcentral.com/da-lite_high_power_screen_review.htm
About $300 for the DaLite Model B high power screen at 100".
Electric non-tab-tensioned electric screens are something which I don't recommend. They are exactly the same as manual screens, but with a motor. Which means you get lousy quality, but end up paying an extra $200 or more to get a motor which allows you to not get off your rear to go pull down the screen. Instead, you get off your rear to go press a button on the wall. Get up and go pull down a manual screen and save your cash for a better screen!!!
Electric tab-tensioned screens are the bees knees when it comes to retractable screens. You get the convenience of a motor (get IR control or a 12v trigger type) and the tensioning characteristics of a permanently tensioned fixed on-wall screen.
Big plus: The screen goes away when not in use. Put it in-ceiling or behind a soffit to maintain a pristine room when not being used and when the screen comes down, you get flat perfection for screen viewing.
Big minus: Price. Period. Also worth noting is that cheaper screens are not always perfectly square and may not be using the highest quality controls or motors which could burn out and fail well before the better motors and screens have issues.
Best match for the HD20? Nothing really works with this type of screen and this lower cost projector. Focupix offers a 100" model which is around $700 shipped...
http://www.htdepot.com/Focupix_TensionFlat_16_9_Electric_Screen_105_p/fxt169s-100.htm
But, the quality and number of reviews is extremely limited (non-existent?) for that model.
Da-Lite and Draper both offer exceptional tensioned screens and they run from about $1,500 upward. You can find some for a bit less, but not much is there.
The screens from Elite also can be good, and run on the cheaper side and should be considered.
At the end of the day, I would try to go with a fixed frame screen and I would strongly recommend Carada for the quality you will get. Yet, for a retractable screen, I would suck it up and get a Da-Lite or perhaps Draper tab-tensioned model. Yes, it costs more than the projector, but it will outlast the projector by years, perhaps decades.