So it seems Brewer's Friend will not let trial members "rebuild" recipes. If you upgrade ($15/year) you would be able to rebuild the recipe to the scale you desire. As#RustyHorn said there is no requirement to change the recipe, it will do fine in your vessel as is. If you are kegging in 5 gallon kegs a 20 liter recipe is about perfect.
But if you are bottling and can manage the mash and boil I say why not brew what fits in your fermentor? Now I don't have Brewer's Friend full version but tried pulling the recipe into Beer Alchemy and scaling it there. I think this would work to get 25 liters into your 30 liter fermenter. That should be ok for a belgian. "Brew Day Instructions" posted below. I'd question the yeast calculation, you probably want to do a starter if you increase the batch size. Don't do this if you can't safely boil 30 liters

Good luck!
Summary
Gravity Before Boil: 1.052 SG (13.4 Brix)
Original Gravity: 1.068 SG (17.2 Brix)
Final Gravity: 1.010 SG (8.4 Brix)
Volume Before Boil: 30.00 l
Volume After Boil: 27.50 l
Volume Transferred: 25.00 l
Water Added To Dilute: 0 ml
Volume At Pitching: 25.00 l
Volume Of Finished Beer: 23.75 l
Total Water Required: 36.67 l
Total Grain: 6.669 kg
Batch Cost: $0.00
Preparation
If using a yeast starter prepare one a couple of days in advance.
If necessary crush the grains.
Heat 20.01 l of water to 61 ˚C.
Mashing
Dough in the following fermentable ingredients.
5.500 kg of German Bohemian Pilsner Malt
412 g of German CaraAmber
206 g of German CaraAmber
206 g of German CaraMunich II
206 g of German CaraAroma
137 g of German Carafa I
Mash pH should be 5.2.
Allow the temperature to stabilize at 55 ˚C. Allow to rest at this temperature for 30 minutes.
Pull a decoction of 5.66 l, raise to boil, and boil for a total of 45 minutes. Add back to mash to raise the temperature to 66 ˚C and then rest at 66 ˚C for 60 minutes.
Raise the temperature to 74 ˚C over 10 minutes and mash out.
Sparge the grains and top up if necessary to obtain 30.00 l of wort (requires 16.66 l).
Boiling
Bring the water to the boil.
When the wort is at the boil add the following fermentable ingredients (stirring to avoid scorching).
550 g of Sugar - Corn Sugar/Dextrose (Dry)
275 g of Sugar - Candi Sugar Amber
Add the following hops at the times given.
29 g of Slovenian Styrian Goldings (60 Min From End)
44 g of Slovenian Styrian Goldings (10 Min From End)
Also add the following during the boil.
3.56 g of Irish Moss
Boil the wort for a total of 90 minutes.
Chill the wort to the appropriate pitching temperature and rack the wort to your fermenter.
Fermentation
The desired volume at pitching is 25.00 l.
Ferment at 22 ˚C with
1 pack of DCL WB-06 Safbrew
If using a secondary, transfer when appropriate.
Racking And Packaging
If using a secondary, the beer should be matured for 4 weeks before racking into either bottles or keg.
If not using a secondary, the beer should be racked directly into bottles or keg and matured for 4 weeks.