andy - you are using an old version of xara and so it might not quite work the same way

you can try approaching the issue the other way round and resize the base graphic [make it smaller presumably ? - it could be a 32bit limitation]

if, in line gallery, you right click on the brush and select copy brush graphic to clipboard it may give you an idea of the base size your program is working at, but you may just have to experiment; make sure you keep your base shape is all vector and grouped

once the brush is made you should be able to resize it for drawing