I need to use a textarea and it must not be resizable, is there an standard of how many lines it could have or maximum size of it? Remember that I just want to make confortable the reading and writing using the least space as possible on screen
You need to make it the length of the expected answer, in the same way that you would for an input box. This helps to guide the user towards the expected answer.
The standard configuration for a textbox is that it is resizable. If you don't want it resized, don't use a textarea. Jakob Nielsen advises against changing how standard elements work.
To add to Yvonne's answer:
1) use common sense to predict the default length and ship it
2) then get data-driven and get the median length of the number of characters entered by Users to a text area and adjust the size. Add some extra space to the calculated median, as Users could be initially constrained with the size from point 1)