Hi guys -
If you upload a FIT file that is pretty big (over 1MB is big for a FIT file), you might cause a crash in the server. This usually happens with really long rides with 1 second recording turned on. 1 second recording is turned on by default if you're using a power meter. I recently had to help someone who had ridden over 700 miles (4 days continuous). His FIT files were 8Mb each, and had MILLIONS of datapoints to parse through. This is more than my tool can handle!
So, here is how I helped him, and how you can work it out yourself.
1) Upload your files one-at-a-time to your Strava account. Make sure to make them private so you don't start getting Kudos and Comments.
2) Once the files are uploaded, Export your individual files as separate GPX files from Strava. The Export GPX option is under the 3 dots by the pencil on your activity page.
3) Zip (compress) all the exported GPX files into one zip file. On a Mac, highlight the GPX files, right click on then, and select "Compress".
4) Upload the resulting zip file to my GPX merger tool.
5) Choose Correct options, and also select “Use Every Fifth Trackpoint” (or fewer/more points, depending on how outrageous your original files were). Don't worry about losing important information. Most Garmin devices on Smart recording take a point every 4 seconds, so if you had 1 second recording turned on, you're basically reducing your file to the size of a "normal" Garmin Track. Human powered activities don't move very far in one second, Heart Rate resolution doesn't change that quickly. Power is about the only measure that can change this quickly, but if you're doing a huge ride, you probably don't need that level of granularity.
6) Wait a bit - takes a while for Strava to process big files!
Hope that helps you with your really big files.
Here is a discussion on other strategies for uploading large GPS files.