Max Screen Size

I want so see if you have any thoughts on how to limit the size of a particle in a Geometry Shader, just like in the Shuriken particle system. How to express the particle in viewport size and limit it.

Code (CSharp):

1.  
2. // Vertex Shader ------------------------------------------------
3.     GS_INPUT VS_Main(appdata v)
4.     {
5.         GS_INPUT output = (GS_INPUT)0;
6.         output.color = v.color;
7.         output.pos =  mul(_Object2World, v.pos);
8.         output.info =  float4(v.info,0,0);
9.         return output;
10.     }
11.    
12.    
13.     // Geometry Shader -----------------------------------------------------
14.     [maxvertexcount(4)]
15.     void GS_Main(point GS_INPUT p[1], inout TriangleStream<FS_INPUT> triStream)
16.     {
17.         //inverse view matrix
18.         float4x4 i_v = inverse(UNITY_MATRIX_V);
19.         float4 pos = p[0].pos;
20.         float distance = length(_WorldSpaceCameraPos - pos);
21.         float3 look = normalize(_WorldSpaceCameraPos - pos);
22.         float3 up = float3(0,1,0);
23.                        
24.         //the rotation matrix
25.         float4x4 rotateM = rotate(float3(0,0,sin(p[0].pos.x) * _Time.y),float4(0,0,0,0));
26.         up = mul(rotateM,up);
27.         //rotate the up vector to match the camera up
28.         up =  mul(i_v,up);
29.         float3 right = cross(up, look);
30.         //individual size of particle
31.         float size = p[0].info.x;
32.                        
33.         //quad vertices centered on 0,0
34.         float4 v[4];
35.         v[0] = float4(right * 0.5f  - up * 0.5f,0);
36.         v[1] = float4(right * 0.5f + up * 0.5f, 0);
37.         v[2] = float4(- right * 0.5f - up * 0.5f, 0);
38.         v[3] = float4(- right * 0.5f + up * 0.5f, 0);
39.                        
40.         float2 u[4];
41.         u[0] = float2(1,0);
42.         u[1] = float2(1,1);
43.         u[2] = float2(0,0);
44.         u[3] = float2(0,1);
45.    
46.         FS_INPUT pIn;
47.         //use the look direction as the normal
48.         pIn.normal = look;
49.         pIn.color = p[0].color;
50.         //store the distance of each particle
51.         pIn.info = float4(distance,0,0,0);
52.                        
53.         for(int i = 0;i < 4;i++)
54.         {
55.             pIn.pos = mul(UNITY_MATRIX_VP,pos + v[i] * size);
56.             pIn.tex0 = u[i];
57.             triStream.Append(pIn);
58.         }
59.     }
60.