The correct adverbial form is “really” rather than “real,” but even that form is generally confined to casual speech, as in “When you complimented me on my speech I felt really great!” To say “real great” instead moves the speaker several steps downscale socially. However “really” is a feeble qualifier. “Wonderful” is an acceptable substitute for “really great” and you can give a definite upscale slant to your speech by adopting the British “really quite wonderful.” Usually, however, it is better to replace the expression altogether with something more precise: “almost seven feet tall” is better than “really tall.” To strive for intensity by repeating “really” as in “that dessert you made was really, really good” demonstrates an impoverished vocabulary.