What about the size of the slot, does that matter??
As long as the card has the same or less size than the slot, it will fit.
Will it work as you want it to? Maybe.
What does matter is whether you will end up disabling or reducing the bandwidth of other slots by adding this card.
Read the manual for your motherboard.
It will tell you what is shared, disabled or otherwise impacted by adding a card, and where you may add it without impacting the rest of your system.
For example. Using the second x16 slot in many boards forces the main video card to run at x8 instead of x16.
Another example is the shared x1 slots that may impact other slots in your machine.
This is often a matter of having more options on a given motherboard than a cpu and bridge chip can handle all at once. Compromises are made. It's up to you to decide how best to allocate limited rescources.
You have the manual. It will tell you what if any impact there is to what you want to do.