Phil Chouinard shares this tip:

You can change your current text size by multiplying or dividing it by the active scale by using these simple keyins:

mdl load calculat;tx=*{tcb->xactscle}
mdl load calculat;tx=/{tcb->xactscle}

For example, if the active text size is 10, and the active scale is 2, then this keyin mdl load calculat;tx=*{tcb->xactscle} will result in a text size of 20.

These keyins work fine in all versions of MicroStation including V8i.