The first step should be to make sure that you know the weight and size of the heaviest load you want to handle and bearing in mind the long life of most forklift trucks, it's worth trying to predict what the situation is likely to be in the medium or longer term and note if things are likely to change.
FORKLIFT CAPACITY. The correct capacity is obviously one that will cater for your needs both now and in the future. There isn't much point in obtaining a forklift larger than needed as it's physical size overall will be bigger and it may not fit the application.
LOAD SIZE is critical as forklift trucks have a dimension known as the load centre which should normally not be exceeded. Most forklift trucks have a load centre of 500 mm, (20 ins), nowadays which means that the length of the load should not exceed 1000 mm. If the load length does exceed this then you need to obtain a forklift truck with a larger capacity. It can easily be worked out what the capacity of the forklift truck should be by doing a simple moment calculation but do make certain by checking with the forklift supplier or manufacturer as there may be some other factor that only they know about.
FORK LENGTH should be, as an absolute minimum, two thirds the length of the load. It's better to have the forks longer than this but care should be taken not to overdo the length to avoid problems encountered by the forklift operators when they have to undercut the load.
LIFT HEIGHT. This is obviously important and again it's best to plan for the future. The important thing to bear in mind is that the higher the forklift stacks, the more the truck's capacity de-rates. This de-rated figure can be obtained from the truck supplier. If you require more capacity at a given height then be prepared for a reduction in the rear tilt dimensions of the mast or it may be that you have to obtain a forklift truck with a larger than needed capacity at ground level in order to have the capacity you require at high levels.
Another feature of lift height is the type of mast that will be fitted to the truck. If the stacking height is fairly low, (three to three and a half meters), the forklift truck will be fitted with a single stage mast often known as a Simplex mast. For higher stacking heights a three stage mast or triplex mast will usually be fitted. Generally speaking a three stage mast will give less driver visibility than a two-stage mast and it will also cost more which of course will be reflected in the forklift truck price.
OVERALL HEIGHT. The overall height of the forklift truck may be affected by the height of the mast when it is closed. if the mast chosen has quite a low overall height then it is likely that the top of the canopy guard will be the highest point on the forklift truck. With higher stacking masts however the dimension to the top of the mast, when fully lowered, becomes the critical point and care should be taken to ensure that the forklift truck will fit through every doorway and under all low ceilings etc.
FREE LIFT. On many forklift trucks, as the forks start to rise, the mast overall height also increases thereby increasing the overall height of the forklift truck. The amount by which the forks can rise from ground level before the overall height of the mast increases is known as the free lift. This can become an important dimension in applications where low headrooms are involved such as mezzanine floors and the like. It is possible to purchase a forklift truck with a so-called full free lift mast. This design of mast allows for the top of the forks, the highest point, to reach the top of the fixed section of the mast without any increase in overall height. This type of mast is sometimes fitted as standard nowadays.