You really want to try some on if you can. Sizing can vary a lot and you do want them to be quite a snug fit so they don't get sucked off in mud. Spike length is easy - buy a range (6, 9, 12 & 15mm covers it) and decide on the day according to course and conditions. You might even still decide to use trail/fell shoes, e.g. if the course includes stony trails rather than soft grass.
I'm a big fan of the Brooks Mach series, version 16 currently in use for xc and track. More roomy than some xc spikes, light and comfy.
You really want to try some on if you can. Sizing can vary a lot and you do want them to be quite a snug fit so they don't get sucked off in mud. Spike length is easy - buy a range (6, 9, 12 & 15mm covers it) and decide on the day according to course and conditions. You might even still decide to use trail/fell shoes, e.g. if the course includes stony trails rather than soft grass.
I'm a big fan of the Brooks Mach series, version 16 currently in use for xc and track. More roomy than some xc spikes, light and comfy.