Would an alien spaceship have to be a certain size - for power carrying, flying saucers, protection etc. - to arrive in the solar system ?

Dependent on the technology used.   Obviously, it will not use SPEED, but a "Physics shortcut" of some sort to get here like a "space warp" or "space Jump"   The latter being Possible by HUT (Holographic Universe Theory)  which is gaining favor quickly in Physics circles. where wave attributes of particles can theoretically  be "Zapped" and "reset" to match another location in the universe even light years away, instantly.  Perhaps in a series of "hops" rather than the entire journey.-------How efficient the system is, how quick, and how economical, would determine the size of the craft.  A cheap, quick journey would have a smaller craft with fewer passengers and fly more often---but if it is slower and expensive, it would be a bigger ship and larger crew.

The distances are overwhelming for our species. Accelerating to and decelerating from the speed of light is a problem for our species. Accelerating at 5g to the speed of light would take over 1700 hours, over 10 weeks! Acceleration requires power/energy we currently can't easily produce. At the speed of light, the nearest star is over 4 years away!  Clearly interstellar travel is far beyond our current ability. My problem is another person and I witnessed a very large triangularly shaped vessel moving silently at treetop level approximately 35 years ago so I know there are flying technologies in use beyond our current ability. Can't prove it so why even mention it?

It would have to be big enough to carry its crew and that would depend on the number, size, and space requirements of the people.

It would have to be big enough to carry whatever power source, resources, food, etc. they would require.

It would depend on how fast the ship travels and how far they travelled.

If aliens from outside our solar system are coming here, clearly they have some far-advanced technology we can't understand.  People like Neil DeGrass Tyson believe we will never leave the solar system.  If others are coming here from light-years away, they probably have figured out how to travel faster than light.

So if these aliens have figured out how to do this, and we don't believe it's even theoretically possible, clearly we have no way to explain how they're doing it.  It could be in a big mother ship, which they'd park somewhere and send out smaller ships.  It could be in a ship the size of a golf ball, the aliens being 1mm tall.  It could be in a ship the size of the Death Star that's hiding in the asteroid belt. Anyone who could answer this question would be guessing!

In Space, the Truth can be Stranger than Fiction

Some great answers here actually

We are not doing too bad ourselves when you think of Quantum Entanglement

Thought up by Stephen Hawking

It has since been Proven !!

Black Holes were once Myths

Though it was hard to get a close look at it, Oumuamua was estimated to be somewhere between the size of a football field and a container ship.

On the show "Star Trek", we see the USS Enterprise as this very large ship... when the camera shows people walking around inside, the corridors are 8 to 10 feet wide, the rooms are relatively spacious, and it doesn't appear very crowded...  it implies that they have power to *splurge* and provide themselves with luxuries... But, the real reason for the wide corridors & spacious rooms was so they could fit the camera and crew in with the actors...

If you compare the above to what we saw in Mercury, Gemini, Apollo - even the Space Shuttle and the Space Station - you see that there's little room and equipment everywhere. They didn't launch any additional mass that didn't have a purpose - even if it meant more room for them to move around in...

My thought is, if aliens are anything like us, they'll send up exactly what they would need to, and splurge a bit for mental health if needed, but no more... especially when you're moving across *light years*  - any additional mass not truly needed for the trip would be left behind, to conserve fuel and power. 

I don't think we've been visited as yet, but a ship - any ship - going to space is going to be equipped with exactly what it needs, and nothing more... How *big* that ship must be, I don't know, as we wouldn't have anything to compare it to; but, my guess is, it would need to be fairly big - especially if it's carrying smaller craft used to enter & exit Earth's atmosphere...

depends on what the alien technology is that gets it here

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.”

Archimedes

If they have enough resources to create such spaceship they will surly meet us

No, a mother ship can arrive just outside of the planet and launch a smaller ship from said mother ship.