Suppose I use a texture rectagle in a shader, eg I've drawn it as

glBegin(GL_QUADS);
    glTexCoord2f(0.0, 0.0); 
    glVertex2f(0.0, 0.0);
    glTexCoord2f(texSize, 0.0); 
    glVertex2f(texSize, 0.0);
    glTexCoord2f(texSize, texSize); 
    glVertex2f(texSize, texSize);
    glTexCoord2f(0.0, texSize); 
    glVertex2f(0.0, texSize);
glEnd()

Then in my shader I want to read an offset from the current pixel in the calculation, for a usual texture we can simply calculate the offset as below:

float offset = 1/texsize
vec2 bl=vec2(tex2D(tex,coords+offset)) +
        vec2(tex2D(tex,coords-offset); 
glFragColor=bl;

However, for a rectangle texture can I simply do the following?

float offset = 1;
vec2 bl=vec2(tex2D(tex,coords+offset)) +
        vec2(tex2D(tex,coords-offset); 
glFragColor=bl;

cheers

That is correct, texels are always 1 unit large in texture rectangles.