Singer 15 and 201 and 66 are all full-size machines. Afaik, all 201-2 made in USA have the "potted" electric motor. There are 201K machines from Britain that can be used in treadles or as handcrank machines, or can have a belt-driven motor bolted onto them. All 201 have the tension on the front of the machine. All have that "paperclip" decal set, to my best recollection. All 201s are wide across, from the pillar to the needle bar. The 201 is the widest in that respect, except for perhaps the 127 shuttle machine.
Singer 201K (the British K models)
http://edsmum.wordpress.com/2010/11/...201k-electric/
Singer 201-2 w/ potted motor
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99 and 99k are 3/4 size and they just look "squattier"
1950s 99k
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the older style 99 / 99k
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vs. the last incarnation of the 66
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15 series are full size and they are the only one that has the thread tension on the side
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Yes, there are measurements but I never learned what they are. Just Google and study the images. Doesn't take long to start being able to ID the model just by sight. The 99 and the 66 are the only ones that I have to look at twice, sometimes, and that's mostly only if the photo is taken from an unusual angle.
I don't remember what is 221 vs. 221-1. Maybe 221-1 is when they stitched from scrolled to striated faceplate?