Resize event Qdialog

If you want the size of the widgets to depend on the father you must use layouts, in this case you must place both frames in a QHBoxLayout. Assuming that in your code you want the width of the right frame to be 900 then you must use setFixedWidth (900).

*. ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Dialog</class>
 <widget class="QDialog" name="Dialog">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>604</width>
    <height>329</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Dialog</string>
  </property>
  <layout class="QHBoxLayout" name="horizontalLayout">
   <property name="spacing">
    <number>0</number>
   </property>
   <item>
    <widget class="QFrame" name="frame">
     <property name="styleSheet">
      <string notr="true">background:red;</string>
     </property>
     <property name="frameShape">
      <enum>QFrame::StyledPanel</enum>
     </property>
     <property name="frameShadow">
      <enum>QFrame::Raised</enum>
     </property>
    </widget>
   </item>
   <item>
    <widget class="QFrame" name="frame_2">
     <property name="styleSheet">
      <string notr="true">background:qlineargradient(spread:pad, x1:0, y1:0, x2:1, y2:0, stop:0 rgba(0, 0, 0, 211), stop:1 rgba(255, 255, 255, 221));</string>
     </property>
     <property name="frameShape">
      <enum>QFrame::StyledPanel</enum>
     </property>
     <property name="frameShadow">
      <enum>QFrame::Raised</enum>
     </property>
    </widget>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>

*. py

from PyQt5 import uic, QtCore, QtWidgets


class Principal(QtWidgets.QDialog):
    def __init__(self):
        QtWidgets.QDialog.__init__(self)
        uic.loadUi("12.ui",self)

        self.setWindowFlags(QtCore.Qt.FramelessWindowHint)
        self.setSizeGripEnabled(True)
        self.setAttribute(QtCore.Qt.WA_TranslucentBackground,True)
        self.frame_2.setFixedWidth(900)
        self.resize(1000, 600)
        self.dragPosition = QtCore.QPoint()
        self.center()

    def mousePressEvent(self, event):
        if event.button() == QtCore.Qt.LeftButton:
            self.dragPosition = event.globalPos() - self.frameGeometry().topLeft()
            event.accept()

    def mouseMoveEvent(self, event):
        if event.buttons() == QtCore.Qt.LeftButton and not self.dragPosition.isNull():
            self.move(event.globalPos() - self.dragPosition)
            event.accept()

    def center(self):
        qRect = self.frameGeometry()
        centerPoint = QtWidgets.QDesktopWidget().availableGeometry().center()
        print(centerPoint)
        qRect.moveCenter(centerPoint)
        self.move(qRect.topLeft())


if __name__ == '__main__':
    import sys

    app = QtWidgets.QApplication(sys.argv)
    p = Principal()
    p.show()
    sys.exit(app.exec_())

Try this:

from PyQt5.QtWidgets import QMainWindow,QApplication,QDialog
from PyQt5 import uic, QtCore

class Principal(QDialog):
    def __init__(self):
        QDialog.__init__(self)
        uic.loadUi("12.ui",self)

        #self.setWindowFlags(QtCore.Qt.FramelessWindowHint)
        self.setSizeGripEnabled(True)
        self.setAttribute(QtCore.Qt.WA_TranslucentBackground,True)
        #self.frame.resize(self.QDialog.width(),self.height())
        self.frame.resize( 1000, 600 )
        self.frame_2.resize(self.frame.width()-900, self.frame.height())

app = QApplication([])
p = Principal()
p.show()
#p.resize(1000,600)
app.exec_()