You say integers, but I suspect you mean positive numbers. Here is a rough quick attempt at a solution:
sed 's/\([0-9]\+\)\s\+\([0-9]\+\)/\1 \2/g'
There are caveats here, such as if the second number starts with a negative sign, or if there are spaces you don't want handled like this that occur between, for instance,
27B. Or if you don't want all numbers to be separated by two spaces. But this is a start that you can modify as you will.
Rereading your question, it appears that all of the spaces you want to "squeeze" occur specifically on lines starting with the word "SIZE". This makes it much simpler. You say "other instances of consecutive spaces" you want to preserve, but it sounds like those are on other lines. So in that case just use:
sed '/^SIZE/ s_\s\s\+_ _'
(There are two spaces in the replacement text.)
Rereading again, I now see you want to fix one space to two spaces. This gets a bit more dependent on the exact format of your lines, but I would say:
sed '/^SIZE/ s_\s\+_ _3'
should do it. (It works on the sample you provide.) The
3 makes the substitution only occur on the 3rd match of
\s\+ - which, as chaos explained, means "one or more space or tab". So in
SIZE<space>:<space>X<space>Y, it will turn the space in between the X and Y into two spaces.