Well if you're looking at specific seats and want to be sure about the side impact spacer, you can usually download manuals without actually buying the product. But in every seat I've come across which has a removable thing, it's removable specifically for that reason, to allow for more space on the non-door side, and doesn't compromise safety to remove it there.
The Safefix can be used up to 150cm if it fits in your car, though I don't think the headrest extends up quite as high as the Kidfix or Besafe Flex fix do - those are both very high extending HBBs.
It's just that basically there is an "invisible box" which denotes the max dimensions for seats within the i-size specification, and that "box" is used by both car manufacturers and car seat manufacturers. So if Citroen for example want to designate certain seating positions as "i-size compatible" then the interior space on that vehicle seat to accommodate a child seat must allow for the whole of this invisible box. And likewise, if a car seat is "i-size compatible" it (and dummy of relevant size) must fit within the invisible box.
When R129 first came out a lot of manufacturers/retailers emphasised the "i-size" part and so many people don't really know what i-size means, and TBH it's not really important to know, but it is a particular thing which is a subset of the newer (R129) regulation and a subset of all isofix seats, mainly to do with vehicle compatability, so the "box" is part of that. It's meant to do away with the need for fit lists.
Britax are saying that when a child is over approx 133cm, the seat/child will exceed the height of the box. And some cars, particularly smaller cars might have a roof which is basically touching the top of the "box", so you could struggle, if you had a smaller car, to fit a taller child in once they have exceeded this. But IIRC the Grand Picasso has a fairly high roof, doesn't it? So it seems to me that this is unlikely to be a problem for you.
I have just looked it up and the max height of a booster to fit into this "box" is 76cm, and both the Safefix and Kidfix extend up to 83cm total height. Besafe izi Flex Fix is up to 82cm total height. An older child will usually exceed the top of the booster by a few centimetres as well.
They actually all have a small print bit in the manual saying you might hit the ceiling in smaller cars with a child over ~135cm, so I don't think this is especially an issue, it was only to be aware that as the Safefix sits the child slightly higher internally, you might hit that point faster. But having looked again, I don't think there's that much in it. Sorry if I was making it sound alarming!
If you do order seats online, just check the returns policy. Under the distance selling act you do get up to 14 days to return anything sold online, but a lot of retailers are understandably not keen on accepting returns on car seats. The law is on your side but depending on the retailer you might have a fight about it.