How do you purchase extra slots in a guild topic I can only see buy a new topic...
Also how much do an extra 25 slots cost?

You'll click on "My Guild" and it's in the Admin Options menu to the left:



You'll be presented with available options/forum gold requirements.

5 slots costs an extra 250

just click on buy a new topic, and on the redirected page you can buy slots too

Sorry "slots" was the wrong term to use
What I meant was increase the guild topic size from the standard 25 to 50.
Sorry for the confusion

you need the appropriate Guild rights to see this setting

I can see everything other than buy extra post size in the guild topic... What extra guild rights would i require?

ask your guild leader, i do not know the Guild option list :).

can you see the edit button right to the guild topics?
it looks like this



if you have the edit buttton, click it and and voila



if you don't have this, your guild leader needs to give you the Edit Settings Rights in the guild topic.