Size And Location
One of the reasons that koi fish are such a popular tattoo design for a sleeve tattoo is the long rectangular shape that fish naturally have and thus works into the canvas of an arm perfectly. Also since the design is going on the arm it is very easy to depict the fish flopping and curving its body in the water and it looks very natural. Thus the size and shape of the koi is perfect for a sleeve tattoo.
The first thing that you have to consider though is the size of sleeve that you want. Are you going for a quarter sleeve tattoo, half sleeve, or full sleeve design? Thus will make a big difference in the size and placement of the koi fish and it is important to talk about this with your tattoo artist.
Historical Symbolism
Aside from being the perfect shape to fit on the arm koi fish are popular for the deep history and symbolism behind them. They are a very traditional symbol in Japanese culture. In fact you can hardly find a temple that does not have a koi pond somewhere on the temple premises and thus shows the important and deeply spiritual and symbolic nature the koi fish holds for the Japanese. They have integrated their love of the koi into tattoos a long time ago. The myth goes that koi fish will swim upstream fighting their way against the current. This shows their strength and power and the koi fish that can make it all the way to the top will pass through the Dragon Gate and become dragons. So it is traditionally a religious symbol for struggling up stream, going against the current of life to achieve spirituality. However, this symbolism holds true in a more secular and non religious way also. It is a symbol for power, and strength.
Modern Symbolism
The symbolism from the ancient past has been adapted to life of today and essentially is the same but the koi has taken on a few new symbolic meanings also. The story of its struggling against the current has meant to those of us that are less religious to symbolize the struggle to get through life and be different and individual. The koi in this sense is going against the status quo or current trends. This was not the original Japanese meaning since they are a very collective culture but as the tattoo has become very popular in the west some of the symbolism has been adapted and changed. It is also a symbol for protection here in the west.