For pixel art tiles or characters etc, pick a size divisible by 8. 8x8,16x16,24x24,32x32,64x64px etc.
For a tilemap, say you choose a tile size of 16x16, make your total tilemap width and height a multiple of that number for example, a 10 x 10 tile map will be 160px x 160px.
In your art software, create a grid guide that is 16x16px so that your drawing canvas is segmented into 16x16px tiles.
Draw your tiles onto your canvas and save the image as a png.
In Construct, add a tilemap object to the game. In the properties set the tile size to 16x16px. In the image editor load in your exported .png file.
Back in the normal editor, stretch your tilempa to cover your viewport/layout or whatever area you want it to cover. Select the tilemap object and use the tilemap editor to select tiles and paint them onto the canvas using the brush tools.
Highly recommend doing some of your own research and watching videos etc. as this is basic stuff, and you'll never learn by not trying it yourself.