The breeding portion of my fleece flock now consists of :-
- one black Wensleydale
- one Manx Loaghtan
- two BFL x Shetland
- three 3/4 Shetland 1/4 BFL
- one Shetland x Manx
- one Shetland
plus, on the candidates for keeping list from last year :
- one 3/4 Shetland 1/4 BFL
- one Shetland x Manx
That's quite a list. 
How much effort do you expend making sure that you pick a compatible breed of ram that wont give you trouble lambing?
I spend a lot of time thinking and researching, and then have to make a choice based on what's available locally!
The only one I felt was a risk this time was the Manx Loaghtan to the Romney. Small sheep, big tup. But the Romney, though large, isn't horned, isn't double-muscled, and is reputedly an easy-lambing breed, much used for crossing on moorland and hill sheep. The Manx, like the Shetland, is a primitive with a wide pelvis, probably wider than many of the moorland and hill sheep routinely used to cross with the Romney. People put Shetlands and Manxes to Texels and Blue-faced Leicesters, so the Romney shouldn't pose even as much of a challenge as a Texel.
Our researches led us to believe that birth size is greatly influenced by the mother's size, whereas eventual size is equally influenced by both parents, or may be bigger than either parent due hybrid vigour.
Also, given our plan to create a larger sheep with nice fleece and all the other attributes, I can't justify DC's place in my flock if she can't manage a bigger lamb. So it was a calculated risk. I only put this one of my smaller sheep to the Romney this time, so I could assess how it goes before putting the other smaller sheep to him another year.
The birth size thing so far seems to have been born out, and on that basis I would happily put the Shetland and Shetland x Manx to the Romney another time.
I hope I will always be able to use a smaller tup for our first-timers, to let them learn their job without too much physical stress. Because it's a palaver having two tups here, my game plan is to breed first timers every other year, so some will be 2 years old and some 3 years old when they first lamb. We shouldn't have any difficulty managing them so that they don't get overfat and unable to get in lamb.
edited to add: And you'll notice the prevalence of Shetland crosses in the list. The Shetland element, I believe, massively reduces the risk of misalliance, in that she has a very wide pelvis, and her lambs are generally born small, active, hungry and determined!