Go anywhere in the middle to upper range of the recommended 4-stroke engine size.

O.S. FS-II series 4-strokes are VERY nice for learning about 4-strokes with. They will spoil you. I went 20 months without needing to touch my needle valve on my .91. (which has the power to take an 11 lb airplane unlimited vertical without using full throttle... so much power... I have to FORCE myself to use more than 2/3 throttle any more.)

MAgnum 4-strokes are much less expensive and everone that I know that has one is very happy with thiers... Probably not bad as a first 4-stroke.

YS and Saito are not as forgiving about needle settings. They generally need a little tweaking with weather changes. They do give more power per cubic inch than the O.S. They are more expensive and repair parts are more expensive... Great engines, but I don't consider them good for a first 4-stroke.