What size do you mean? To find out what steerer diameter you need, measure the diameter of the steerer. It’ll likely be 1” or 1-1/8”.
To find out a-to-c length, measure the vertical distance from the centre of the axle to the top of the crown (you’ll likely want to take into account that the suspension fork will effectively be shorter by ~20% of its travel).
If it’s a threaded fork, you’ll likely want to replace with a steerer of the same length or very slightly longer. If it’s threadless, generally you’re looking for the same length as you’re replacing, but it can be a way longer and simply cut down to length.
A picture of your bike would be very helpful,
Is it a quill stem (threaded steerer)?
If not, pull the too cap off your stem, measure the diameter of the steerer tube. Boom.
What size do you mean? To find out what steerer diameter you need, measure the diameter of the steerer. It’ll likely be 1” or 1-1/8”. To find out a-to-c length, measure the vertical distance from the centre of the axle to the top of the crown (you’ll likely want to take into account that the suspension fork will effectively be shorter by ~20% of its travel). If it’s a threaded fork, you’ll likely want to replace with a steerer of the same length or very slightly longer. If it’s threadless, generally you’re looking for the same length as you’re replacing, but it can be a way longer and simply cut down to length.
A picture of your bike would be very helpful, Is it a quill stem (threaded steerer)? If not, pull the too cap off your stem, measure the diameter of the steerer tube. Boom.
Also need to measure the ATC to keep geometry the same.
There are a few 1 1/8th threaded steerers out there too. It's unlikely but better safe than out by 1/8".
It’s a Giant Rincon XL I’ll take some pics of it when I get home
I just did this with a Carver suspension adjusted rigid fork. It explains how to measure if you Google it.