I want to make aerobatic plane with wingspan about 50" (I wanted to make 30" one, but changed my mind because I was told that it would be too expansive, and hard to make). Can anybody tel me what is traditional speed for this type of planes?

For an example plane of that size, take a look at Gary Wright's E3D. 48" wingspan:

http://www.gwmp.net

For examples of smaller aerobatic models in the 30" wingspan size:

http://www.mountainmodels.com

Speed can be anything you want. It depends on the motor and wing design. Wing span has no direct relationship to speed.

My aerobatic e planes range from 15 to 30 inch spans, and all are simple (cheap) brushed setups. My D.U.F.F. is 28 inch S300 geared that 3D's on 7 600AE's (plane will tie itself up in knots), my Nuthin' Extra won't 3D but handles most aerobatics, it's 30 inch, all pink foam. S400. On 4 600AE's it's a good aileron trainer that'll loop and roll, on 8 600AE's it really rocks. My 15 inch Acro Bipe uses the GWS Lite Stick motor DD and it's a hoot, just need to drink a lot of coffee before I fly it.

Mike

Ha Ha Ha. I loved this line about a lot of coffee. I am going to have to remember this one.