GridView items: set relative sizes

Question

I'm trying to display some items in a GridView. I want each item to fill a relative part of the screen (image B) and all I have is GridView take just the space it needs at the top of the screen (image A). The image:

(I want each item sizes over 15-20% height of the main layout)

EDIT : I found this question Percentage width in a RelativeLayout. 'LadaRaider' gives a good answer but I don't know how to use that solution in my GridView cells. This is my layout.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:orientation="vertical"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
   android:background="@drawable/background">

 <TextView
   android:id="@+id/label_name"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:textAppearance="?android:attr/textAppearanceLarge"
   android:gravity="center_horizontal"/>


 <LinearLayout
     android:id="@+id/ll_ok"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
     android:orientation="vertical" >

     <TextView
         android:id="@+id/textView1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="@string/lbl_choose"
         android:textAppearance="?android:attr/textAppearanceMedium" />

      <GridView
     android:id="@+id/grid"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
     android:numColumns="2"
     android:columnWidth="100dp"
     android:stretchMode="columnWidth"/>
 </LinearLayout>

 <LinearLayout
     android:id="@+id/ll_wait"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
     android:gravity="center"
     android:orientation="vertical" >

     <TextView
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="@string/lbl_wait"
         android:gravity="center"
         android:textAppearance="?android:attr/textAppearanceMedium" />
 </LinearLayout>
 </LinearLayout>

I have also created a custom layout to use in GridView items. It has just a TextView and a background.