Space calculation for a dataset is a simple task, yet can go wrong on many levels. In your case, you've given insufficient information to provide a clean solution.
To really understand the concepts of it, you'll have to read the manuals (such as 'Using Data Sets
' for starters and then moving onto 'Using IBM 3390 Direct Access Storage in an MVS Environment
'), though you can refer this topic
for a quick reckoner. Skip to Expat's post, which gives a detailed explanation as to how you can arrive onto a value for Cylinder allocation; furthermore if you read the manuals stated earlier in this post, you'll come to know about the concept of 'extent', 'multi-volume dataset', 'format=large/extended' and so on- all of which play important role in dataset storage usage.
Read through, understand the concept, use it to come up with an allocation; if you're stuck at any point, let us know where and we can guide you.
Edit: Sorry Steve didn't see your post, I think I started the reply earlier hence missed the post.