issues scaling three.js sprite texture

I am trying to scale a sprite and fill the entire screen, to have text on the top and bottom of the screen, but when I do so, it scales the text also. So how do I put a big line of text at top of screen??

my code:

var ui_canvas = document.createElement('canvas');
    var size = 1028; 
    canvas.width = size;
    canvas.height = size;
    var context = ui_canvas.getContext('2d');
    context.fillStyle = '#ff0000'; 
    context.textAlign = 'center';
    context.font = '48px Arial';
    context.fillText("some text", size / 12, size / 12);
    context.strokeStyle = '#ff0000';
    context.lineWidth = 1;
    context.beginPath(); 
    context.moveTo(0,0); 
    context.lineTo(size, 0);
    context.lineTo(size, size);
    context.lineTo(0, size);
    context.lineTo(0, 0);
    context.stroke();

    var amap = new THREE.Texture(ui_canvas);
    amap.needsUpdate = true;
    amap.generateMipmaps = false;

    var mat = new THREE.SpriteMaterial({
    map: amap,
    depthTest: false,//to keep always in front
    depthWrite: false,//to keep always in front
    transparent: false,
    sizeAttenuation: false,
    useScreenCoordinates: false,
    color: 0xffffff
    });

    var sp = new THREE.Sprite(mat);
    sp.scale.set( 1,1,1 ); // CHANGED
    //camera.add(sp);
    //sp.quaternion.copy(camera.quaternion);
    sp.position.set(-10, -6, 0);
    scene.add(sp); 

sp.position.set(-10, -6, 0); does nothing, since sizeAttenuation = false, but even moving the sprite closer when sizeAttenuation = true, the text just gets bigger. I want more area to write text.

sp.scale.set( 1,1,1 ); also grows the text.

and making the sprite bigger and putting smaller font ex: context.font = ’12px Arial’;
makes poor resolution and unreadable font.

how to solve this? how to have high res, crisp font and text all over the entire screen?

thank you!