Question

How to setup(in XML) that the button will have width half of screen. I found only wrap content, match parent(fills whole screen) and exact amount of dp eg: 50dp.
How to set it exactly hold the screen?

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
 android:weightSum="2"
>

 <Button
     android:id="@+id/buttonCollect"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:layout_weight="1"
     android:paddingLeft="8dp"
     android:paddingRight="8dp"
     android:text="przycisk" />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:text="Button" />

Answer 1

This can be done by having two widgets on your layout:
use a LinearLayout and set layout_width="fill_parent" on both widgets (Button and another widget ), and set layout_weight also to the same value .
and the LinearLayout will split the width between the two widget equally and your button will occupy half of the screen.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="fill_parent"
 android:layout_height="wrap_content"
 android:orientation="horizontal">

 <Button
     android:id="@+id/buttonCollect"
     android:layout_width="fill_parent"
     android:layout_height="wrap_content"
     android:layout_weight="1"
     android:paddingLeft="8dp"
     android:paddingRight="8dp"
     android:text="przycisk" />

<Button
    android:id="@+id/button2"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_weight="1"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:text="Button" />