Reflectors are as effective as their coating, size, and the light source allow - for example, they are virtually useless in complete darkness, but quite good in sunny conditions.

Some reflectors offer multiple coatings in one set, e.g. black, white, gold, silver, and often a diffusor as a frame to hold the coated cover. I usually go for white, the diffusor, or gold, but that of course depends on the shot at hand. They are relatively cheap and come in all sizes, so to try things out, you can go for them, as the are not too bad.

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As to "what size and shape is optimal?":

As a rule of thumb: the bigger, the better. The biggest sensible size of course depends on environmental factors. E.g. if you do a lot of hiking, you will probably use a smaller reflector, as you will most likely encounter winds in the mountains. If you have 1-2 assistants, I would go for the biggest reflector I can afford. This of course ignores situations like reflecting only on a certain spot (e.g. the face), which will most likely be easier with a smaller reflector.

The form will depend on what you are trying to achieve. If you want to enhance larger areas, then personally, I would go for rectangular ones. If you need something like a spot, then round ones will get the job done.

Note that there are other forms, as well, like triangles:

Image stolen from Manfrotto's website

...of which Manfrotto's marketing department says:

Replacing the need for a regular shaped reflector the TriGrip reflectors utilise a triangular design which provides a much stronger and more stable structure. They also feature a moulded handle with securing strap, allowing accurate positioning with one hand. This means light can be reflected into awkward spots that would've previously required a stand system.

So there are endless varieties of reflectors, all of which (seem to) have their purpose. Personally, I still go with different sizes of cheap, round, 5-in-1 reflectors, because I do not use them too often, because I have other financial priorities at the moment, and because they still work.