Text Box Expand option

Hello Everyone,

I am new in SSRS. I would like to ask about expand option of text box. I have a text box when we click on that + sign then it shows two other text box which i created. But i want to change the size of that + sign. I tried alot and search alot on internet but not getting any answer.

Can anyone tell me how can i increase the size of that + sign?

Thank you in advance.