The size of a reflector can have an impact on the outcome of your pictures, but overall its affect is limited. Generally speaking, the larger the reflector, the more light is spread out.
Reflectors come in a few different shapes, but the shape of a reflector has no affect on the overall results the reflector produces. Shapes range from circular to oval to triangular, but what you choose is largely a matter of personal preference.
The color of your reflector is the most important aspect to consider when purchasing or using a reflector. Reflectors come in many different colors. Below is a short outline of what effects each color can produce.
Gold: Use gold reflectors to warm up your pictures.
Silver: Silver reflectors can be used to brighten your picture, without affecting the color of the light.
Silver and gold stripe: Similar to the gold reflector, the silver and gold striped reflector will warm up your pictures. However, the effect it produces is softer, while still warm. This produces a flattering warm light, ideal for portraits.
White: Use white reflectors to bounce light into shadows, without affecting the quanity, quality, or color of light being cast on your subject.
Translucent: Translucent reflectors can be used to soften light, and are often placed above your subject.
Black: Black reflectors act in a subtractive manner, removing excess light from your photos.
Blue: Blue reflectors are often included in reflector kits, but in reality, they are not reflectors. Instead, they are useful as Chroma Key digital backgrounds for background replacements.
Green: Like blue reflectors, green ones are also included in several kits online. They are also used in digital photography for making background replacements.
If you will be moving around often, portability is a factor to consider. Hand-held reflectors can be useful when on the go, as they have a convenient handle and do not require stands in order for you to use them.