When using three.js with an orthographic camera, the size of a Sprite can be magnified or reduced when zooming the mouse. To prevent the text size from changing, consider adjusting the camera settings and applying proper texture and material to the Sprite.
var camera = new OrthographicCamera(width / - 2, width / 2, height / 2, height / - 2, 0.01, 100000);
var texture = new THREE.Texture( canvas );
var material = new THREE.SpriteMaterial ( { map: texture, transparent:false } );
var sprite = new THREE.Sprite( material );
scene.add(sprite);
Need help keeping the text size consistent when zooming the mouse? Here's a possible solution.