What is the biggest tyres size which will fit on my front rim?

[janekbyd]

Hi.
I would like to fit to my K100 retro tyres. 
 According manual recommended tyre size is:
- Front: 100/90 R18 (3.5 - 18), Rim size: 2.5x18
- Rear 130/90 R17 (5,1 - 17). Rim size: 2.75x17
Is there any chance to fit on a front rim 4.25 or 4.5 tyre?
I would like to reduce visual difference between size of front and rear tyre. Like on this photo.
(I know that BMW R... has got both rims in the same size) ;-)
Cheers Marcin

[Laitch]

There are a couple of manufacturers with 110 fronts. Continental is one of them.

[The Mighty Gryphon]

Unless you change the triple clamps you will be limited by the space between the fork legs.  A 110/90 front tire will fit the 2.5" rim and is 4.3" wide.