The phrases
the same size
and
the same sizes
both have the right to exist, and are used to mean different characteristics of the objects which they describe.
If a class of objects can be dimensioned in two or more of ways, for example, pipe segments have the outer (or inner) diameter, wall thickness and the length, then, comparing two pipe segments, you can say
Those two segments have the same sizes.
if you mean that all dimensions of the two objects measure equal to each other (within some tolerance).
If in some context only specific dimension (like length, for instance) is deemed important, and other dimensions can be ignored, you can say about two pipe segments of two diameters
Those two segments have the same size.
if their length is what's currently important.
When talking of items of clothing, you can say of different pairs of shoes that they all have the same size, although no dimension of those complex shapes is the same if carefully measured: the shoe "size" is a marking, classifier, which is only approximately associated with a certain dimension.
Back to your example...
A filled sphere (a ball) has essentially only one dimension - the diameter. Often enough it is a hollow sphere, and the wall thickness can be taken as the other dimension, but if it is a solid sphere, there is no other dimension. This way when you only compare one dimension, it is more natural to use singular. But it would not be wrong to say
The diameters of those balls are [all] the same.
They have the same diameters.
Phrase "the same" when describing a dimension means "of the same value".
Here, some means
some
1. being an undetermined or unspecified one
So there isn't an issue regarding plurality: justification can be singular.
You can use justification as a plural like you would with good reason.
Here is an example in the same sense as your example. I don't know why there were budget cuts. There must be some good reason for the recent budget cuts →
- There must be some justification for the recent budget cuts.
There can also be multiple undetermined justifications. I don't know why there were budget cuts. There must be some good reasons for the recent budget cuts →
- There must be some justifications for the recent budget cuts.
If there is just one justification for a particular action, then justification (singular) is correct.
- We have just one justification for the recent budget cuts.
If there are more than one reasons for a particular action, then justifications (plural) is correct. For example, imagine that there are ten reasons for the budget cuts, but only three are presented: here are some reasons for the recent budget cuts→
- Here are some justifications for the recent budget cuts.
Here's a headline I found online that uses justifications.
18 Solid Justifications for Cloud Computing -- and 10 Situations Where It Doesn't Work
Best Answer
This is one of those strange exceptions to the rule in English - it's not correct, but it's well understood by native speakers.
Normally, you would expect the plural to be used - multiple weddings for multiple couples - but it doesn't scan as well when read/spoken. This occasionally results in conflicting plural/singular for a handful of phrases where it 'feels' nicer to say it in a particular way.
However, in the example, while it refers to multiple brides and grooms, it refers to their singular wedding day. Readers would interpret this sentence to understand multiple couples and their respective single day.
Strictly, the singular day could be shared by all the people or could be a different day for each person. However in this sentence one considers each person as an individual because it's very unusual to have a shared group wedding ceremony.
It may also be inferring some brides and some grooms, in various combinations. The unfortunate thing about this particular example is that it requires some awareness of recent history in context of marriage practice.
In a pure grammatical sense, the start of the sentence is wrong. For clarity, I would write,
The use of "brides and grooms" in the example implies that most/all unmarried couples behaved in this manner, and readers would understand this given context. While it's not grammatically correct, it's one of those evolving aspects of language we can't really stop people from using!