Text line

Right-clicking on the line... if there is one ;-)
Select a measure, open the line palette, do double ckic e.g. on Crescendo. Then, as mentioned above, right click on the line etc. etc.

Style/Text->Stave

These are not text lines; text lines are what you see at the bottom of the Lines palette. Ctrl+T is staff text. Another word for staff is stave (US vs UK mostly).

But you shouldn't need to do this at all. You can instead simply change the size of all measure numbers using Style / Text / Measure Number. Or, if you wish to still have small measure numebrs but also have larger marks in certain places, don't use staff text for this - that actually is confusing since it isn't at all clear to someone reading the score that this is what those numbers represent. Instead, use Rehearsal Marks (Ctrl+M) - those are the standard way to mark special places in the score. You can either use letters A, B, C etc or numbers 1, 2, 3, etc or just type the measure number. Either way, it will appear not only large but also with a box around it, which is the commonly accepted notation that everyone will recognize. Plus it will break multimeasure rests, appear on all parts, and other behave the way you need it to behave.