Example of a TextField without any padding.

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'TextField without padding',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(primarySwatch: Colors.blue),
      home: Material(
        child: Center(
          child: Container(
            width: 150.0,
            color: Colors.lightGreenAccent,
            child: TextField(
              decoration: InputDecoration(
                contentPadding: EdgeInsets.all(0.0),
                isDense: true,
                border: InputBorder.none,
                hintText: "Type a message",
                hintStyle: TextStyle(
                  color: Colors.grey,
                  fontSize: 14.0,
                ),
              ),
              minLines: 1,
              maxLines: 1,
            ),
          ),
        ),
      ),
    );
  }
}