Standard Size or Custom Fit
The very first consideration you need to bare in mind is whether you'll need a custom fit loft hatch to fit your exact requirements or whether you can size your loft hatch based on a standard size measurement. Of course this very much depends on what you intend to use the loft for. If you plan to store large items in your loft it stands to reason that you'll need to install a loft hatch that can accommodate the largest item you plan to house in there. The type of ladder you use will also have an impact on the size of your loft hatch. If you favour a concertina fold-down loft ladder then you will obviously need a loft hatch that allows for the ladder to pass through at its longest and widest point. Loft ladders and loft hatches are often sold in packages which pairs loft ladders with the appropriate hatch.
Loft hatches are available in a variety of materials. The most popular material has always been wood because it's light, cheap and can be adjusted if it doesn't fit exactly. Wood is still the most popular choice of material for custom made loft hatches, for obvious reasons, with PVC the second most popular selection. Custom fit PVC loft hatches are catching up quick however, due to the quick lead times for delivery and their low prices. The situation in the standard size loft hatches market is very similar. Wood previously dominated but now metal loft hatches are catching up in the commercial arena due to their fire resistant qualities, and PVC is increasing in popularity in the domestic market. The benefit of wood, however, is that it can be refinished and repainted time and time again which makes it a much more flexible option. Your redecoration options could be limited with a PVC loft hatch because although they are available in a variety of colours, once you buy it that colour it has to stay that colour.
The location of your loft entrance will have a big bearing on the type of loft hatch you need. If the entrance is located near a wall then you'll have to make sure that the loft door doesn't swing down and mark the wall. You'll also have to make sure that the presence of the wall doesn't interfere with the loft ladder, either by creating an awkward angle or by not allowing an internal ladder to extend out fully.
If your loft entrance is located above a stairwell this will have significant implications to the type of loft hatch you choose. In this situation there are limited ladder configurations available to you, so you'd be forced to choose from a small selection of loft hatches that suit this scenario.
For a loft hatch located in a very cramped space you may wish to consider a hatch that opens inwards rather than outwards. Inwards opening loft hatches are easier to open but more difficult to closer than outwards opening alternatives but they also pose fewer problems when trying to maneuver in a small area.