Marklin 24530-1 - C-Track Curved Track, R-5, Box of 6

Radius R5 = 643.6 mm / 25-5/16". Curve 30°. Parallel curve to Radius R4 with a spacing of 64.3 mm / 2-9/16".

12 sections of track form a circle with an outer diameter of 133 cm / 52-3/8".

Charlie's Tip: This is the largest (widest) curved C-track. The only other option if you need something even wider is to use the 24912 Track pieces. They do not really match any of the R-1 to R-5 geometry, but are used (normally) in combination with the very wide radius turnouts 24711 and 24712. 

Read Charlie's blog on C-track curves