"waterline" are ship that are modeled from the water up (hence the name). Waterline ships are fine (in some respects easier as you don't have to worry about the lower hull, screws and props), but don't look right unless you display them in a diorama.
As for scale, just like with other types of models, the scale is how many scale feet per actual feet. Your 1/350 Tamiya Enterprise example is about 37 inches long. The Revell's 1/720 Enterprise is about half that. As for popular size it's a toss-up. 1/700 are cheaper & easier to display. However, 1/350 are easier to build do to the parts being larger and easier to see and work with (IMO). What it all boils down to is personal preferance. Personally I try to work in 1/350, but that's my choice.
As for a first ship I would stay away from the big "E". Tamiya makes an awsome 1/350 Fletcher class destroyer that while I haven't done it (yet), I hear nothing but GREAT things about it. It's only about 15 inches long (not too big).
I would try the Fletcher and a 1/700 ship of your choosing and see which you like best. Try looking thru this ship section and you will learn alot.
Tim H.
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