1. Within the vertex shader, add the following uniforms:

uniform float MinParticleSize = 0.1; 
uniform float MaxParticleSize = 2.5; 

2. Also, within the vertex shader, in the render function, we'll update the size of the particle based on its age:

void render() {
    Transp = 0.0;
    vec3 posCam = vec3(0.0);
    if( VertexAge >= 0.0 ) {
        float agePct = VertexAge / ParticleLifetime;
        Transp = clamp(1.0 - agePct, 0, 1);
        posCam =
            (MV * vec4(VertexPosition,1)).xyz +
            offsets[gl_VertexID] *
            mix(MinParticleSize, MaxParticleSize, agePct);
    }
    TexCoord = texCoords[gl_VertexID];
    gl_Position = Proj * vec4(posCam,1);
}