I just got a Fenix 8 amoled and using it mounted on a kayak but the screen is to dim in the sun light. What can I do?

You should have bought the Solar.

It’s to big. I only bought the watch because I could get the small 43 with same size screen as the Fenix 6 Pro.

For me it is the other way around, the Amoled screen gives too much light when I am in the sun during the day, but in the evening when it gets dark I can no longer read the data from the screen because the Amoled screen becomes too dark.

It’s ok when it isn’t dimmed. It sounds a little wired. Do you have the latest update11.60?

Same reason as to why I won't buy AMOLED Fenix for mounting on the bars of the bike as the screen being unable to run at full brightness due to battery drain. MIP displays (and maybe micro LED in future) are the right tool for the job. AMOLED is not. It's really only meant to be used in "raise to wake" type situations.

I use Epix 2 51mm for more then one year on my MTB without any problems. Setting: rise to wake active and longest dim timeout (15sec). Battery drain ca. 4%/hour.

Garmin don't want to allow longer timeout or completely disabling dim timeout due the risk of pixel burn in.

How do you then activate the watch on the handlebar?

Activate the watch? Maybe you mean active the full brightness ... MTB = shock and shaking all the time, which works as gesture. But I also don't see the dimmed display as a big problem. Sure, sometimes it's too dark, then I touch the display for full brightness.