Hi Ben,
Before I go on I just want to say there are other people on this forum that know alot more about these carbs than I do but I will try to help as best I can. In response to your question the easy response is 40's are big enough for a stock L-series motor with alot of people installing 32mm chokes to maintain drivability down low. It's as much a question of what chokes you install as it is how big you go on the carbs, ie you can buy 45mm carbs but choke them down to say 34mm chokes or you could go the other way and buy 40mm carbs and install 34mm chokes getting a very similar result.
There is so much to be said on this topic that would go way beyond my knowledge base so I try and stick to the basics as I understand it based on research and trial and error.
With choke sizes just keep in mind that the size influences air velocity which means smaller chokes gives more air velocity pulled into the combustion chamber when inlet valve opens. If you have big carbs you will get slower velocity at low revs, these set ups are good for engines with hot cams that have a higher power band as with alot of track cars, (ie they spend most of the time at the 3500rpm and above range so works well for that sort of driving). When you have chosen the size of carbs you want then you will need to select the right size idle jets, for idle and progression, right size main jets and emulsion tubes for mid to upper range. I have prepared a spreadsheet based on my reading and set up, look in the carbs section, it's not going to tell you which carbs to buy but might help in starting.
I am pretty confident that I have a very good starting point for an L26 or L28 motor and that is:
Dellorto DHLA40's with:
32mm chokes
55 idles
40ish pump jets
125 to 130 mains
7772.6 (or .7) emulsion tubes
refer to the spread sheet in the carbs section for other details, ie main venturi, air bleed on transition circuit etc.
If you want to go down the Dellorto path send me a PM and Ill put you onto a guy in Italy who will sell you all three already rekitted for a really good price, (you will then need to buy manifold $300, linkage kit $200, soft mounts $75.00, velocity stacks $225 from memory and filters prolly about $150 for a set) . So before you start to tune them it will cost you about $1500 (+ all the new jets you will need as you tune). You can tune them yourself to a cetain degree. I must admit I'm pretty happy with how mine go, better than the SU's with no hesitation anywhere like some people have had issues with. It has cost many people I have spoken to around $2000 to $2500 all set up including tuning on a rolloing road.
Sulio