Question for R800 triple

[Enlight25]

I have a R800 triple Caad 4 built in 2000.
The tires are 650c and just wondering if anyone would know if 28’s will fit? 23c are on there now and 25c will fit but fitting 28c would really make a more comfortable ride.

Also, does anyone know the diameter of the seat post?

thanks

 

[Trailmix]

Not sure about the tire clearance but older Cannondales used 27.2mm seatposts.