antenna size ?

Different designs will result in different lengths and spacings. That's the easy and short answer to your question. No one design is the best of the best as there are just too many variables to deal with such as gain, FB ratio, bandwidth, then you get into available tubing sizes, one piece or tapered tubing, element spacings to achieve best gain, or impedance matching etc etc etc. You can either beat your head against the wall trying to design your own from scratch without benefit of an antenna test range or you can simple copy a proven design where all the extreme math has already been done for you.