making a photo big enough to print A1 size

A1 is pretty big at 23.4 x 33.1 in.

Having no idea what you're starting with for size, we can't really say how big you can make your image before it turns into pixelated junk.

Standard output resolution is 300 dpi. The person you spoke to may be able to use a lower resolution image, but when you get below 200, pixelation starts getting readily apparent. Most online sites that make prints from your uploaded images only work with 300 dpi. If you send them something with a lower res than that, their system will automatically scale the image up so it will print on their devices.

I mention this mostly because the site léonie linked to will only process an image to a maximum of 4500 x 4500 pixels. At 300 dpi, that's only 15" square. Well below A1 dimensions.

Give us an idea what you have. Open the image in Preview. Press Command+I. What does it say for the current pixel dimensions and dpi? Such as this example for a small image I opened.

I would upload the photo to the Online Image Enlarger.

See this link: https://www.imageenlarger.com

There you can increase the pixel size of the photo without making it blurry.

The site offers several methods to increase the number of pixels. Pick the one that looks best to you.

What is the current size of your photo - the width and height in pixels ?

The photo appears to be 1024 x 600.

Okay, that rates as teeny tiny. Looks nice on a monitor, is drastically too small for A1. It's literally impossible to scale an image up that far without it looking like it's made up of fuzzy squares.

At 300 dpi, that's a 1,170% increase, and will result in a roughly 241 MB file. At 200 dpi, that's a 780% increase and about a 107 MB file. Either way, this will not be a pretty image.

Sorry, no. The image you have is simply too small to have any chance of going that big and having it look good. There's nothing to stop you from making it that size if you want, but you can get an idea of how blocky/fuzzy it will look by scaling it in Preview.

Open the image and choose Tools > Adjust Size. In the sheet that drops down, make sure Resample image is on. Also Scale proportionally. Change the size drop down to whatever seems easier. Inches makes the most sense here. Change the smaller width to 23.4. The other dimension will follow (you'll notice it comes out to about 39").

When Preview finishes scaling the image, you'll be able to see what it will look like. I'm afraid you won't be impressed.

Still not good. At the 300 dpi setting the image is currently at, it's only 2" x 3.413". Taking it to A3 (11.7 x 16.5 in) at the same resolution is a 585% increase.

Don't believe the person you were talking to that they only need a couple of more megabytes. Yes, you can send the image to a printer with the resolution set really low (like a typical 72 dpi image), but the print will have the same blocky/fuzzy affect as scaling it up at 300 dpi.

There's no way around the issue of having nowhere near enough pixel data to make a print much larger than the 2" x 3.413" it currently is, and still look sharp. A 200% increase is the most you could hope to get away with.

That photo is really very small at 1024 x 600.

A photo may still look decent when printed at 250 dpi, but then you will need 8275 x 5850 pixel.

So you need to enlarge the numbers of pixel by factor 8 to 10.

Even with the smart algorithms of the image enlarger I would not go beyond a scale factor of 3 or 4. Otherwise the image will still look sharp, but unnatural.