In our club we have a Christmas cup, with different boats

In a regatta with different boat's lengths
which is the best way to determine the length of the starting line?

We can think with 10 kts and with 20 kts wind.

Thanks !!!!

Lenngth of the longest boat x 2 x number of boats

I was going to suggest taking the average length x 2 x # boats.  Longest boat is OK also.  I think it also depends on how aggressive the fleet it, current, etc

Mk 1 eyeball and experience - or a laser rangefinder, as used in golf
And what length do you want - well it depends. But most calculations produce a line which can be impracticably long for a large fleet. Ideal, but remember that there will probably be a couple of ranks of boats on the line. (If a club race they may not even be near the line, whereas in a championship fleet they may all be on the line - like I said - it depends.

At the 10 knot end of that range I’d use 1 1/4 or 1 1/2 as the multiplier, rather than 2. Range finder (from pin boat shooting at signal boat) very helpful.

Well, to determine the length of the starting line you need to use this formula:
 
Starting line length = Number of boats * Average length of all boats * Multiplying factor

In your specific case, where the boats are different types and  you have to choose a Multiplying Factor from 1.1 to 2 considering the level of agility in the water of the boats (in case of skiff boats up to 3). 
In case of strong wind, increase the Multiplying Factor but without exceeding 3.

Add all length of all boats and then you may take the multiplying factor 1,5. 

Onya Andreas, I agree.

Are you having one start or multiple starts?  - usually similar boats would start together (for example 30 to 35 feet in length/similar rating)   In that case you would do all the math suggested above for the biggest division you have.

Another approach with a fleet with all different kinds of boats is a Pursuit Race - where the line does not have to be that line as only one or a couple of boats are starting at a given time based on their rating.

Depends on fleet type but this was something myself and another PRO came up with for important events, it does deviate from US Sailing's RC class recommendations.  First the key for what abbreviations mean in the spreadsheet, then the calculation, then two example outputs with wind measured in knots and lengths in meters, second table is in feet.  I typically will do this with the entry scratch sheet and print it out for my notebook before heading out that morning.  Summary is that in light air a boat gets 1.1 times the length of the boat all the way to heavy air where the boat gets 2.9 times the length of the boat where I assume high waves due to where I race.

Attached is a spreadsheet of the below examples so you can change it to your preferences and feel free to add a current modifier.  It was virus free when I attached it, no macros are in it.  Change the first ABL (B11) cell and NSB (B12) (highlighted in yellow) to get the rest to update automatically.  If you want to use US feet enter the foot length in ABL (B13 or B21) and the rest will update appropriately.

SLL : Start Line Length
WSA : Wind Speed Average = (WS+Gust)/2
ABL : Average Boat Length
NSB : Number of Starting Boats
LLPB : Line Length Per Boat
Lenght in meters, wind in knots.
SLL = ABL * ( WSA + 7 ) / 12) * ( NSB + 1 )

In metric measurement.
SLL  | 210  | 229  | 248  | 267  | 286  | 305  | 325  | 344  | 363  | 382  | 401  | 420  | 439  | 458  | 477  | 496  | 515  | 535  | 554  | 573  | 592
WSA  | 5  | 6  | 7  | 8  | 9  | 10  | 11  | 12  | 13  | 14  | 15  | 16  | 17  | 18  | 19  | 20  | 21  | 22  | 23  | 24  | 25
ABL  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10  | 10
NSB  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20
LLPB  | 11  | 11  | 12  | 13  | 14  | 15  | 16  | 17  | 18  | 19  | 20  | 21  | 22  | 23  | 24  | 25  | 26  | 27  | 28  | 29  | 30

In foot measurement.
SLL  | 630  | 687  | 745  | 802  | 859  | 916  | 974  | 1,031  | 1,088  | 1,145  | 1,203  | 1,260  | 1,317  | 1,375  | 1,432  | 1,489  | 1,546  | 1,604  | 1,661  | 1,718  | 1,775
WSA  | 5  | 6  | 7  | 8  | 9  | 10  | 11  | 12  | 13  | 14  | 15  | 16  | 17  | 18  | 19  | 20  | 21  | 22  | 23  | 24  | 25
ABL  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30  | 30
NSB  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20  | 20
LLPB  | 32  | 34  | 37  | 40  | 43  | 46  | 49  | 52  | 54  | 57  | 60  | 63  | 66  | 69  | 72  | 74  | 77  | 80  | 83  | 86  | 89

☒