Growing large crystals can be a challenging and intricate process, and it often requires careful control over many factors. If you've already experimented with material ratios, nucleation space, and reaction time without success, here are additional strategies you may consider to increase crystal size:

• Look into scientific literature for specific methods used to grow large crystals of your particular substance.
• Consulting with a crystallographer or material scientist with experience with the specific material can provide valuable insights tailored to your situation.

Remember, crystal growth is highly material-specific, and what works well for one substance might not suit another. Careful observation, patience, systematic experimentation, and a willingness to delve into the specifics of the material and the crystallization process are often necessary to grow larger crystals.

What material are you talking about? In general terms, growing crystals contains a significant amount of art and craftsmanship. Therefore, general recommendations may not be effective.

Crystal growth (linear) is a function of several variables, the most important one is supersaturation, which is also a function of several process variables.

Supersaturation should be carefully controlled close to, but safely below the nucleation limit in order to prevent formation of new crystals.

In the sugar industry (with probably the largest rate of crystal production) sugar is crystallized by seeding the sugar syrup with the correct amount of seed crystals in a vacuum pan.

Besides supersaturation, crystal growth depends on temperature, feed syrup purity, crystal content and massecuite circulation.

On-line data on supersaturation, the most important parameter of crystallization is hard to provide for the process control. The only instruments available and used in quite many sugar plants all over the world are the SeedMaster family of instruments, used to provide the supersaturation and other important data for the control of crystallisation in vacuum pans.

For more information on the subject please find my papers on ResearchGate.

To increase crystal size, explore temperature adjustments, employ slow cooling or evaporation methods, consider seed crystallization with appropriately sized seeds (it is very important). In addition, you could test the effect of different solvents, control agitation levels, and explore repetitive or seeded batch crystallization.