hard to say installed. if the machine never had work, there is your answer. if not, probably will have to remove them.
if you are replacing, do not buy till you have the old guides out so you can measure both the guide and bore. if you heat the head in an oven to 400*f, they will come out easily and sometimes, they do not remove material so a std guide fits without issues. just let the head cool naturally in the oven without heat.
have you measured stem clearance? a lot of people scoff at knurling but you can extend the life of your guide that is in the head. like an oil debate, some say it will last longer due to the voids holding oil, but it has been around practically since internal combustion engines have been around, did a few myself on blocks where the guide is the block.

 

While I cannot give you the proper size I like to remove the old first to see what's needed, then get the correct ones. I just run a drill down the guide that will leave about 0.030 - 0.040 thickness left of the old guide. Then you can tap the left over part out without any materiel being pulled from the head itself.

 

The guys I know that port heads typically drive +1 guides into stock heads that have not been handled previously. I've been doing it without issue. (2-3) sets.. Stock hole size is 9/16..

 

 

use a +.001 guide,i have to ask though,why are you replacing them? plus you do realise the guides will need to be honed to size & seats will need to be cut after install,right? really best left to a shop with the proper tooling & experiance