Originally Posted by
AdelaaR
Any tyre wider than 23 will fit and even a 23 may, so you have plenty of choice there.
For doing both fast road and offroad you should be looking for "cyclocross" tyres, which are specificaly designed for this purpose.
Cyclocross tyres are designed for a variety of conditions, just like MTB tyres. Some are for mud, some are for hardpack, some are for gravel. And they tend to be 35mm, which is too wide to fit on most models of the Sirrus - its narrow tyre clearance makes it a poor choice as off roader.