How to make flutter app responsive according to different screen size?

Using MediaQuery class:

MediaQueryData queryData;
queryData = MediaQuery.of(context);

MediaQuery: Establishes a subtree in which media queries resolve
to the given data.

MediaQueryData: Information about a piece of media (e.g., a
window).

To get Device Pixel Ratio:

queryData.devicePixelRatio

To get width and height of the device screen:

queryData.size.width
queryData.size.height

To get text scale factor:

queryData.textScaleFactor

Using AspectRatio class:

From doc:

A widget that attempts to size the child to a specific aspect ratio.

The widget first tries the largest width permitted by the layout
constraints. The height of the widget is determined by applying the
given aspect ratio to the width, expressed as a ratio of width to
height.

For example, a 16:9 width:height aspect ratio would have a value of
16.0/9.0. If the maximum width is infinite, the initial width is determined by applying the aspect ratio to the maximum height.

Now consider a second example, this time with an aspect ratio of 2.0
and layout constraints that require the width to be between 0.0 and
100.0 and the height to be between 0.0 and 100.0. We’ll select a width of 100.0 (the biggest allowed) and a height of 50.0 (to match the
aspect ratio).

//example
new Center(
 child: new AspectRatio(
  aspectRatio: 100 / 100,
  child: new Container(
    decoration: new BoxDecoration(
      shape: BoxShape.rectangle,
      color: Colors.orange,
      )
    ),
  ),
),

Also you can use:

• LayoutBuilder
• FittedBox
• CustomMultiChildLayout