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I was told over the phone that the bag could be max 158 cm long?
Any experience with this? I was going for a 5'6 surfboard, but might have to switch down to something shorter.

Check airline regulations on their website specifically and print it out to take with you.
1. surfboards might cost extra but bags might allow more weight (99lbs) for the same price.
2. you might try insisting these are your snowboards and let them prove otherwise as snowboards might not have size limit but weight limit only
3. 158cm could be sum of length, width and thickness - read it carefully

The 158 centimetres is indeed the sum. I can not find any information on that applying to sports equipment too though, rather than just the standard bag.
Most big sports equipment would not be transportable if that would be the case, and they specifically mention hanggliding, bikes etc., which would never be packable with a sum of 158 cm.

Length up to 1.99m, weight up to 20kg for a surfbag,

Sports equipment fee: 30 Euro one way for short flights, 60 Euro one way for long flights, so say if you're flying from a European destination to a place like Mauritius via Istanbul, it's gonna be 90(30+60) Euro one way, 180 Euro return, just for the boardbag (that's just one example, they have a special table to calculate other routes)

You may be allowed up to 2 pieces of checked in luggage (Economy), each up to 23kg, and hand luggage up to 8kg.

There are some special routes where Turkish offer special deals, you can carry way more for less, between Brasil and SEA etc, check their website for more info.