[Inkscape-user] Can't change the size of a flowed text

Late of create a flowed text. Appear this object:

https://0x0.st/oSYC.png

When i go to back to text tool, appear this blue box:

https://0x0.st/oSYF.png

I can modify this blue box to modify the allowed size of the text box. Now the problem is the following: How can I modify this size of the blue box manually in pixels? That is, modifying the XYWH values in numbers of pixels.