Both options sound fine to me.
You may want to consider if the middle is the best place and there may be a darker area you want to light.
Lastly consider what will go below the lantern (dining table? Kitchen?). Remember that hanging pendant lights below a roof lantern is difficult and looks odd. One partial solution is coloured LED strips around the edge of the lantern.

Hi PG our new dining room is 5m x 3.5m and our lantern is 2.5 x 1m, I have posted a photo in case it helps. Although your room is a lot bigger still think it will look good with size you suggest either 1200 or 1500.

Thank you everyone - that is really helpful.

Hi

Here are a few photos of roof lanterns we have installed and their sizes.

XACT Pure Glass Roof Lantern - 2140 x 1145mm.

XACT Aluminium Roof Lantern is 1500 x 2000mm.

XACT Aluminium Roof Lanterns - 3000 x 2000mm (L) and 2000 x 1000mm (R),

Also, we are at the Surrey Homebuilding Show on July 1st/2nd where we will be displaying a 1500 x 2500mm XACT Aluminium Roof Lantern and the XACT Pure Glass Roof Lantern. Here's a link for 2 free tickets.

I hope this helps!

xactbp.co.uk

That is so helpful, thank you very much.

Hello PG. Your question is an interesting one! With
regard to the size, it really depends on the look you wish to create. If you're
looking to create an orangery style extension which is very popular at the
moment, a larger lantern would be worth considering particularly from the
perspective of adding the ‘wow factor’. However if the idea is to add a little
light, then the smaller sizes you mention would certainly work. It's perhaps
worth highlighting that the sliding folding doors will add light into the front
of the extension but if you're looking to add light into adjacent rooms, a
larger lantern would again be worth considering. You are very welcome to
send your plans to us and we would be pleased to advise further. The
link below is a project in London, the size of the
longer roof lanterns are 2.4m x 1.2m. Hopefully
this will give you an indication of the light created by a roof lantern of this
size. Also like this example, you might like to consider more than one roof lantern to help define areas. We offer roof lanterns in
purely aluminium and a combination of timber and aluminium.

https://www.houzz.co.uk/projects/1427135/wrap-around-extension-featuring-3no-roof-lantern-installations-in-london

Thank you so much. fantastic to see the photos. I'm thinking 1200 x 2500 is probably big enough as is just to get some more light into the room. Don't want it taking over the room. Silly question please but is the 1200 x 2500 the amount of glass you see inside or is that the overall size from the outside? thanks again.

Not a silly question at all! We work to internal aperture sizes i.e. the hole in the roof.

You might find the Gallery section on our website useful too. It shows lots of projects with their corresponding aperture sizes: Roof Lantern Gallery

All look lovely. Can you please provide me with a quote for aluminium - contemporary so I guess linear rather than classic? 1200 x 2500 - white on white and anthracite grey on white please. Delivery to Essex. Supply only. Thank you.

We're delighted to have the opportunity to quote for you. I'll be in touch with the price ASAP.

Hi PG. Please could you email me at hello@justrooflanterns.co.uk so I can forward you our price. Many thanks, Annabel

Thanks I have just emailed you. Many thanks

Hi PG !
I've just skimmed though the thread - glad you have had some great feedback ! Just wanted to add - if you work out the interior layout before you place the roof light - that can help it tie up with what's below it beautifully !
Rather than placing it in a spot that may or may not work with what's below !

great idea, thank you

Hi Everyone,

We are planning an orangery extension which will combine small seating area and a dining area coming off a large extended kitchen. The only kitchen window is coming out to make doorway through to orangery. So we need light in the kitchen..... will have 5m bifold doors but want to know thoughts on size of the lantern.... will have an ultrasky thin profile max glass with good U values ... but how big.... we want wow but also to be liveable... max could be 6x4 lantern... too big?