Talla is correct in the first sentence.
The error in the second sentence is valla.
Valla should be spelt Vaya. This is a common error among native speakers due to the fact that the sound of "ll" and "y" are the same.
The correct phrase is:
Todavía no estoy segura. Mi mamá quiere que yo vaya a medirme esta tarde.
I hope this helps! I'm a native speaker.