you can define the desired dimension of your canvas through the width or height property.

<canvas id="chartcontainer" width="400"></canvas>

...or using css.

canvas {
  width: 400px;

further you should also define responsive: false and aspectratio: 1 within the chart options, latter represents a square canvas.

options: {
  responsive: false,
  aspectratio: 1,

see configuration options at the chart.js documentation

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