The read-only XRWebGLLayer property framebufferWidth specifies the width of the framebuffer, in pixels.

You can get the height of the framebuffer using the framebufferHeight property.

The width in pixels of the XR device's framebuffer. Each of the framebuffer's attachments (pixel, depth, color, and/or stencil buffers, for example) are all this many pixels wide.

This snippet fetches the framebuffer's width and height for later use.

let glLayer = xrSession.renderState.baseLayer;
gl.bindFramebuffer(gl.FRAMEBUFFER, glLayer.framebuffer);

frameWidth = glLayer.framebufferWidth;
frameHeight = glLayer.framebufferHeight;

• WebXR Device API