What are some physicochemical factors which affect oral sustained-release dosage form design?

They include:

Dose size: For orally administered drugs, there is usually an upper limit for the size of the dose. It is usually 1g for tablets. If the size is over this limit, an alternative solution may be needed (eg. taking more than one tablet instead).

Ionisation and solubility: Compounds which a solubility less than 0.01mg/mL will be sustained. The lower solubility limit for drugs formulated for sustained release is around 0.1mg/mL.

Partition coefficient: Compounds with a poor availability include those with a low partition coefficient.

Stability: Some drugs are not stable in the acidic environment of the stomach and will be released there. It is hence ideal to delay the release of these drugs so that they are released in the small intestine for more absorption.