I would like to know how to create a slider to change the text size – Flutter

Note: your question wasn’t too clear if you’re looking to achieve the same design or regarding the divisions. This answer is how to achieve the same design.

Result

Customization

To customize the Slider to your liking, you can use SliderTheme.
Then, to customize the Tick, you can create your own class that extends SliderTickMarkShape.

Code

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Test()));
}

class Test extends StatefulWidget {
  const Test({Key? key}) : super(key: key);

  @override
  State<Test> createState() => _TestState();
}

class _TestState extends State<Test> {
  var value = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: SizedBox(
          height: 200,
          width: 500,
          child: Card(
            margin: const EdgeInsets.all(10),
            color: const Color(0xff323237),
            child: Padding(
              padding: const EdgeInsets.all(8.0),
              child: Row(
                mainAxisSize: MainAxisSize.min,
                children: [
                  const Text(
                    "あ",
                    style: TextStyle(color: Colors.grey),
                  ),
                  Expanded(
                    child: SliderTheme(
                      data: SliderTheme.of(context).copyWith(
                        tickMarkShape: CustomTick(),
                        trackHeight: 10.0,
                        minThumbSeparation: 2,
                        thumbColor: Colors.white,
                        thumbShape: const RoundSliderThumbShape(
                          enabledThumbRadius: 20,
                        ),
                        inactiveTickMarkColor: Colors.grey,
                        inactiveTrackColor: Colors.grey,
                        activeTrackColor: Colors.grey,
                      ),
                      child: Slider(
                        min: 1,
                        max: 100,
                        value: value.toDouble(),
                        divisions: 12,
                        onChanged: (newValue) {
                          setState(() {
                            value = newValue.round();
                          });
                        },
                      ),
                    ),
                  ),
                  const Text(
                    "あ",
                    style: TextStyle(color: Colors.grey),
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class CustomTick extends SliderTickMarkShape {
  @override
  Size getPreferredSize(
      {required SliderThemeData sliderTheme, required bool isEnabled}) {
    return const Size(10, 10);
  }

  @override
  void paint(PaintingContext context, Offset center,
      {required RenderBox parentBox,
      required SliderThemeData sliderTheme,
      required Animation<double> enableAnimation,
      required Offset thumbCenter,
      required bool isEnabled,
      required TextDirection textDirection}) {
    final Canvas canvas = context.canvas;
    final Paint paint = Paint()..color = Colors.grey;

    canvas.drawRect(
        Rect.fromCenter(center: center, width: 5, height: 20), paint);
  }
}

See also

How can I customize Slider widget in Flutter?