When a new Space is placed in to the 3D model it doesn't use the label chosen in the Annotate Tool Settings dialog, but instead uses a generic space label "artifact". This document shows methods to place the specified automatically, and also how to control the "artifact" label scale.
If you would like to retain the Space "artifact" but simply want to change its scale, you can do so by choosing Workspace > Preferences > Building Designer > Auto Dimensions Font Size > Overall Width & Overall Height and setting appropriate values.