My experience with Linak is that they are CRAP!! Poor build, and bad engineering. Among other things they use PLASTIC for load bearing parts, and do NOT have 'End-of-travel' limits, so they can easily be overdriven. (Supposedly, they depend on detecting excess current draw in the controller circuitry, but this isn't real reliable and can cause additional problems...)
I've gotten some actuators from Progressive Automation, and have been favorably impressed by them.
As BM mentioned you need to know
Voltage
Stroke length.
Shortest length from mounting to mounting.
Newton's or pounds force required minimum
Acceptable speed.
10 newton is 1 kg or 2.2lb. Ish...
Voltage can be anything, but 12 or 24VDC is usually best for chair use. If buying new, this often can be specified (but may cost a lot extra - Progressive does 12V stock, 24V is usually about $50 extra) If controlling the actuator with chair controller electronics (i.e. using the joystick) then you need to match what the controller puts out. OTOH, if using a toggle switch, then you can either use 24V directly, or go through a DC-DC (Pick one with an output current greater than the actuators maximum rating) to get the voltage needed for the actuator.
Stroke length is almost always going to be 2X the minimum length, plus some amount for the gearbox parts, and mounting bits - this should be in the data sheets. Since in many cases, the same gearbox and such can be used to make several different models with different stroke lengths by swapping out the driven shaft and tube, you will often see the lengths described as 'X plus stroke length"
i.e. 3" plus stroke would mean 9" minimum for a 6" stroke, or 39" minimum for a 36" stroke.
You also need to pay attention to the orientation of the mounting eyes with respect to the motors, and the motor mounting position, and make sure that you don't interfere with any other hardware. Some actuators can have their mounting eye orientation changed, others can't...
In general, you need to look at all the dimensions for clearance, and how you will mount the part, but the open / closed lengths are the critical ones.
For any given actuator size, there is a usually a trade-off between power and speed - more power = slower speed. Within some limits, you can get slower speed (and less force) by lowering the voltage, but it is risky to try to get more speed / force by increasing voltage, as you are looking at a motor that is working pretty hard at it's rated output.
Also as you'd expect, more force at a given speed will also mean a bigger motor and shaft diameter
In looking at the force, you need to consider both the 'dynamic' force, or how hard the motor can push / pull (note that this CAN be different for push and pull, but usually isn't) and the 'static' force, or how much the actuator can hold when there is no power on the motor - typically this will be higher than the static force, but not always.
In addition, you MUST consider the leverage and angles that the actuator will be working through, as the amount of force needed to lift the same weight will vary drastically with a change in the lever length - how to do this is more than I'd want to cover in a short post, but is critical to getting something that works.
I also like to insist at a minimum on having 'end-of-travel' switches at the travel limits, and if the actual travel allowed / desired on the device is less than the mechanical travel of the actuator then additionally adding switches at the desired travel stop points - this is especially critical if it isn't immediately obvious when at the end of travel, as it prevents burning out the actuator motor, as well as possible mechanical damage.
ex-Gooserider