I am designing a sign measuring 0.9m x 1.50 m. This sign will be seen from up close and needs to be top quality, high res. However, I need to keep file size manageable. Any suggestions?
There are many good ways of shrinking down your Photoshop document.
Put a Solid Color Layer on the upper most layer
Put a solid color layer as upper most layer. This will greatly reduce file size because Photoshop won't save an high quality preview image inside the .psd file.
Flatten the document
Sometimes you want to send a .psd file but you don't want the other person to have that degree of control over your file as you do. You can simplify your document by merging layers together. This can be useful for layers that contain much detail but merge into a greater concept anyway (for example a sky. the other user might not need to edit every piece of cloud while you want to keep the opportunity open to you).
Delete unused layers
Some layers of yours are probably hidden and never will be used again. Sometimes i catch myself to make many copies of a single layer to create for example shadow effects. You can merge those into one single layer.
Delete unseen material
Sometimes you have a big image inside of an Smartobject. parts of the image might get lost because you resized and dragged them and thus aren't in the border of your canvas anymore or they might have been hidden in chains of multiple Smartobjects when you further refined the image.
An easy way of dealing with this problem is to just rasterize Smartobjects and thus remove potential chains and the parts that are not inside of your canvas.
Get rid of layer masks
They're very useful but still count as layers and thus take up space on your Harddrive. You can right click on them and click "apply layer mask" to remove the layer mask while keeping the effect on the layer
Use adjustment layers instead of blending layers together
You duplicate the space the layer needs if you duplicate the layer and then apply an blending mode to the upper one. You can save space by using Adjustment layers for this. You can also use Smartobjects to only save one image while having 2 instances of that layer interacting with each other. So instead of duplicating layers you should make a Smartobject out of that layer and then duplicate the Smartobject. Other than regular layers a Smartobject only contains a link to another embedded document which keeps the image file.
Don't use images as gradients or solid colors
There are many gradients you can download as image file in the internet. While this might be useful for some resource-light webdesign it's an absolute pain inside of an Photoshop document because Photoshop can easily generate those gradients. This will also have the advantage of having more control over the gradient colors and the flow of them.
Also do not use raster layers for big color areas. That's a job for solid color layers. Photoshop can save them much more efficient.
Turn off "Maximize Photoshop Compatibility"
By using this option, Photoshop will save additional informations for other programs to better interpret the Photoshop document. You can safely turn this option off most of the times.
Compress the .psd file before sending
This is a very neat little trick because it doesn't require you to change your document. Just take a .zip or .rar software of your choosing and compress the .psd document. This will also have an Smartobject-like effect on your image files inside the Photoshop document. This means that you'll ghreatly save space if you have image duplicates inside of your document.
Your canvas size is 0.9 m x 1.50 m at 1:1. Don't work in full scale, you can use vector software and work in a smaller scale.
What is your concept of top quality? I hope is not insane resolution.
1) 100 ppi is good enough on photographs.
2) For text, use vectors in Ilustrator, Indesign or Corel.