You're welcome. You will need the seat post diameter. It is sometimes inscribed on the post but in that condition possibly not visible. If not then you need a vernier caliper or similar. Measure in mm, most sizes are even decimals - 26.8, 27.2, etc. Once you have that, measure the length in mm to where you had it set, add about 100mm. Google (XXmm) long seat post or check with LBS.