If the store has floor stock of your bike ... ask the store if you can get them to measure the post height at full extension for you
measure from the bottom of the expansion slot on the frame to the bottom of the rails
this gives you max height at full extension as an eg 255mm
Then for your chosen dropper get the max length + minimum insertion line (and max insert depth)
Eg for a 385/125 LEV DX the dimensions are
110mm min insert
220 max insert
So the effective lengths are
Max length @ min insert => 385 - 110 = 275mm
Max length @ max insert => 385 - 220 = 165mm
+/- 5mm and depending on how long the frames expansion slot is.
So as for the example with ideal post length of 255mm and presuming the frame can take at least 135mm of post then the 385/125 will suit as 255 falls within the effective post range of 165 and 275mm ... note the difference in static post you will be carrying when the post travel is dropped.
Other considerations are whether you like your post slammed when dropped or prefer to carry a bit of post ... adjust your chosen post length accordingly. eg if you want it slammed you may want to opt for a longer post and travel ... if you like a bit of post then you may opt for a short post with less drop, but carry more of the static post above the seat clamp
eg on my trail bike I was running a 335/100 ... whilst on my AM I ran a 385/125 ... have subsequently gone 125 and 150 respectively as I want to clear steeper stuff at the cost of carrying a little more weight and having to micro adjust post height with my remote more often which comes naturally with practice